Of

cumstances where the body is continually losing, we may Absorption. infer, that something has been gained by absorption. And where the egesta exceed the ingesta in a proportion much greater than the waste of the body will explain, there absorption must have been going on.

* Ratio Medendi, part iv.

chap. 3. + Elem. Physiol.

tom. v. p. 90.

The case of Mr M, published by Dr Currie, is not singular. The writings of physicians abound in similar examples. They had often occurred to that excellent clinical practitioner De Haen, who was therefore persuaded that water was imbibed *. Haller too, with his usual industry, has collected a great many observations of the same kind †.

Again, when physicians were engaged in their extensive statical experiments, weighing themselves, their ingesta and egesta, for many months, nay for years together; they sometimes observed, that so far from losing, they had gained weight, especially during cold and moist weather. Thus, Rye, under a cold and humid atmosphere, gained 13 ounces. Linnings, during two hours exposed to cold, acquired 84 ounces. The abbé Fontana, after two hours exposure to a moist atmosphere, returned home some ounces heavier than he went out. De Gorter gained 6 ounces in one night; and on other occasions, two ounces and four ounces. These observations are confirmed also by the experience of Dr Francis Home, professor of Materia Medica in the university of Edinburgh. Having fatigued myself pretty much (says he) in the afternoon, I went to bed without supper, and was so hungry that I could not fall asleep for some time. Betwixt eleven at night and seven next + Medical morning I had gained two ounces '

Tracts and Exper. p. 250.

Here then are examples of the body gaining considerably more than the ingesta will account for, acquiring weight when neither food nor drink had been swallowed. And we have the concurring testimony of the most respectable writers supporting the same truth.

How can this increase of weight be accounted for, unless by absorption? In such experiments, the loss of weight, which cannot be accounted for by the sensible egesta, is attributed to the exhalation; the increased weight sometimes observed, and which cannot be explained by the sensible ingesta, must in like manner be referred to the inhalation.

on.

That the system may be affected by active medicines introduced and absorbed by the skin, cannot be denied. And were proofs still wanting to establish the doctrine of cutaneous absorption, this argument might be insisted It is true, that friction is commonly employed when we wish to introduce medicines by the skin, by which, it is said, the substance is mechanically forced through the cuticle, and brought into contact with the absorbents of the true skin. The system, however, may be affected without friction, for example, by wearing a mercurial plaster, and more certainly by mercurial fumigation, as practised by Lalonette and others .

Edin. Med. and It might even be concluded from an examination of Surg. Jour. the structure of the skin, that absorption must take place vol i. p. 181. at its surface. We know that the cuticle is porous, and is penetrated by exhaling vessels; we know that lymphatics commence immediately below it; and we know that when certain substances are applied to the cuticle, especially when this application is aided by moderate friction, as in the case of applying garlic poultices to the feet, and the more familiar instances of mercurial in

unction, that these substances are taken up by the absorbents, and conveyed into the circulation.

From a consideration of all these circumstances, we think it fully proved, that the skin is an absorbing or inhaling organ. For further proofs we may refer our readers to Bichat's Anat. Gener. tom. iv. p. 691.

Of Absorption.

189

Mr Charles Darwin, son of the late Dr Darwin, Supposed published in 1780, a Latin thesis, which is translated retrograde in the 29th sect. of his father's Zoonomia, vol. i. in action of the lym which he attempts to prove, that the valves of the phatics. lymphatics are so formed as in particular cases to admit of the regurgitation of the absorbed fluids. The arguments on which he founds this opinion (beside the diffi culty of accounting for the phenomena of several diseases on any other principle), are chiefly the following:

First, The mouths of the lymphatics seem to allow water to pass through them after death, the inverted way, more easily than in the natural direction.

Secondly, In some diseases, as diabetes and serofula, it is probable the valves themselves are diseased, and are thence incapable of preventing the return of the fluids they should support.

Thirdly, There are valves in other parts of the body, analogous to those of the absorbent system, which are liable, when diseased, to regurgitate their contents.

Fourthly, The capillary vessels, which must be considered as analogous to absorbents, may be seen, in animals submitted to the microscope, to regurgitate their contents into the arteries, during the struggles of the dying animal.

By means of this hypothesis (for notwithstanding the arguments adduced in its favour, we can call it, no better) Mr Darwin explained the speedy passage of watery fluids from the stomach to the urinary bladder; the phenomena of diabetes; diarrhoea, dropsy, cold sweats; translations of matter, chyle, milk, and urine; the operation of external remedies, &c.

In all those classes of animals that possess a complete Absorption absorbent system, the phenomena of absorption seem to in the lowproceed much in the same manner as in man; but in est classes some of the inferior animals, especially in mollusca and of animals. worms, this function seems to be performed by the veins. In the echinodermata, however, especially in * Cuvier the sea-urchin (echinus esculentus), lymphatics have Leçons, been demonstrated by the second Monro. In insects tom. iv. and polypes there is no proper absorption.

p. 161. 191

The absorbing vessels of plants are chiefly the fibrils Absorption of the roots, which evidently imbibe moisture, and per- of plants. haps gaseous fluids, from the earth; but there have also been demonstrated vessels opening beneath the outer bark, which botanical physiologists consider as lymphaties. "Lymphatic vessels (says Wildenow), are found in the epidermis of plants, and are of great minuteness, anastomosing in various ways through small intermediate branches." They surround the apertures of the cuticle, by which the inhalation and exhalation of vegetables are carried on; but they are so minute as not to have yet been filled with coloured liquids. Round each opening, which is commonly shut by a moveable valve, they form a circle, rarely a rhombus, as in the zea mays. In the lilium calcedonicum, those vessels run obliquely, and somewhat in an irregular undulating manner, fig. 2. In the ccccxvIII, common onion (allium cepa), they run in a straight, though oblique and regular form, fig. 3. In the pink,

(dianthus

Plate

Of

(dianthus caryophyllus), they are very straight, with Absorption. straight and horizontally transverse branches, fig. 4. In almost every plant they have their certain and peculiar direction, which in each remains constantly the same

Wilde

now's Princ. of

Bot. sect. 236.

192 Theory of absorption.

The theory of absorption is still but imperfectly understood. We shall briefly notice the principal hypotheses that have been brought forward to account for the entrance of substances into the mouths of the absorbing vessels; for.this part of the process alone seems to be disputed.

It was long the opinion of physiologists, that fluids enter the mouths of the absorbents on the principle of capillary attraction; but as the absorbing vessels are not circumstanced like rigid capillary tubes, immersed in a fluid; and as, were this hypothesis just, absorption should go on with regularity, and all fluids should be absorbed indiscriminately, neither of which circumstances is true; this hypothesis is now generally abandoned.

Mr John Hunter accounted for the entrance of substances into the mouths of the absorbents, by attribute ing to the mouths of these vessels a power of nibbling, similar to that exerted by a caterpillar when feeding on a leaf. This opinion may be called ingenious, but is certainly very wild and fanciful.

Dr Fullarton, in a thesis on absorption, published at Glasgow in 1800, attributes to the absorbing orifices, a power of suction similar to that of the proboscis of butterflies, and the tentacula of some marine animals.

The first and third of these hypotheses suppose the absorbents incapable of taking up any but fluid substances; but we well know that even the hardest bones are somehow absorbed, and conveyed to the circulation. We must therefore add another hypothesis, which supposes that there is secreted in the animal body, a fluid + Duncan's capable of dissolving flesh and bones +. The hypotheMed. Com. sis, however, is without foundation. It is contrary to the simplicity of the animal economy, that there should take place, first, a secretion of solid matters to compose the structure of the body, and then a second secretion to dissolve them. We must therefore consider this part of physiology as a subject of future investigation.

vol. x.

¿P. 354.

193 Relations of absorp

⚫tions.

When the absorbed substances have once passed the barrier of the first valve, it is not difficult to account for their further progress through the lymphatics. A continual succession of fresh absorbed matter must drive for ward what is already in the vessels, while the valves must in general prevent its reflux. The impelling force from behind is also probably assisted by the irritability of the vessels. It is not easy to decide how the absorbed matters pass through the glands: if, as is generally believed, they do not in these form continuous tubes, there must be a fresh absorption by the mouths of the vessels that out of the gland, which is as difficult to account pass for as the first reception at the origin of the vessels.

The relations that take place between absorption and the preceding functions, are perhaps the least understood tion with of any that occur in the animal economy. We know the preced- that the action of the absorbents is greatly assisted by ing func- muscular motion, and that it is in general most defective in indolent and sedentary persons. An evident sympathy is also observed between the stomach and the absorbents. Nausea, and especially vomiting, are powerful promoters of absorption, and some remarkable instances are related of the speedy absorption of a large 3

WE have traced the nutritious particles of the food Circula from the intestines through the lacteals, the mesenteric tion glands, and the thoracic duct, into the left subclavian vein, where we find the chyle is mixed with the venous blood, and carried to the right auricle of the heart. We must now consider how the fluids are conveyed to every part of the body, or we must examine the func tion of circulation.

195

This function takes place in all the vertebral animals, Organs. and in mollusca, worms, and crustacea; but there appears to be no real circulation in insects, zoophytes, and plants.

The organs by which circulation is performed differ essentially in the several classes of animals. Those of the human system have been described in the first part of ANATOMY, sect. x. and xi. and a brief comparative view of these organs in the inferior animals, has been given in the second part of that article, No 154, 201204, 300, &c. and in the articies CETOLOGY, ErpeTOLOGY, ICHTHYOLOGY, and ORNITHOLOGY. For a fuller account of these latter, we may refer to Cuvier's Leçons, leç. xxiv. and xxv. and to the Comparative Anatomy of Blumenbach, chap. xii.

196

It is well known that, in the red-blooded animals, the Pulmonic blood is not of the same shade of red in every part of and syste mic parts. the body; but that what has passed through the lungs," and is circulating through the arteries that proceed from the aorta, is of a florid red colour, while that which is sent to the lungs by the pulmonary artery, as well as that which is returning from the extremities of the arteries through the veins, is of a purple or crimson colour. As one set of organs always contains florid, and another set always crimson blood, it is convenient to distinguish each set by an appropriate name. Dr Barclay has done this, and he calls that set of organs which are employed to convey the blood from the arteries, and distribute it to the lungs, pulmonic, comprising the pul monic veins, viz. the vena cava and its branches, pulmonic auricle, pulmonic ventricle, and pulmonic arteries: while he denominates that set of organs which return the blood from the lungs, and distribute it to the system, systemic, comprehending the systemic veins (pulmonary veins), systemic auricle, systemic ventricle, and syste mic arteries, (the aorta and its ramifications). One great advantage of this nomenclature, is that it prevents the ambiguity of the expressions right and left, anterior and posterior, applied to the auricles and ventricles of the heart. We shall therefore employ them in the subsequent part of this chapter *.

*See Bar

Muscular

For an account of the nature and properties of the lay on blood, see ANATOMY, Part I. sect. 14. and CHEMISTRY, N° 2642.

It may not be improper, in this chapter, to notice the principal arguments that have been used to prove circulation of the blood. They are as follow:

the

Motions, p. 231. 197 Proofs of the circula tion of the 1. When the artery is tied, the part of the artery that blood. is betwixt the ligament and the heart, swells; but that part of it which is betwixt the ligature and the remote branches, becomes more flaccid than before. On the other hand, when a vein is tied, the part between the li

gature

tion.

Of Circula- gature and the remote branches, swells, while the part between the ligature and the heart becomes flaccid. 2. The valves placed at the mouths of the aorta and pulmonary artery, must prevent the blood from regurgitating into the ventricles, while they permit it to flow forward through the arteries, into the capillary branches of the veins. Again, the valves situated in the course of the veins prevent the blood from flowing back into the arteries, while they permit it to proceed forward through the venous trunks into the heart.

man.

198

3. By the assistance of a microscope, the blood may be seen in the pellucid parts of animals, as the feet of frogs, flowing from the arteries into the veins.

4. When an artery of moderate size is wounded, and not secured by ligature or compression in proper time, the blood flows out till the animal be dead.

5. Any thin liquid, when injected into an artery, does not pass backwards into the heart, but flows forward into the inosculating veins. On the other hand, such a liquid thrown into a vein, flows towards the heart, and not into the smaller branches of the vein.

The phenomena of circulation in the human body have been already mentioned under ANATOMY, sect. xii. and xiii. and under MEDICINE, N° 95. We shall here only offer a compendious view of the course of the blood in the adult and in the foetal state. Circulation I. After birth, the blood coming from every part of in the adult the body through the numerous ramifications of the vena cava, is poured into the right or pulmonic auricle of the heart, by the contraction of which it is thrown into the right or pulmonic ventricle, which contracting, throws it into the pulmonary artery, being prevented from regurgitating into the auricle by the action of the tricuspid valves. It is now, by means of the ramifications of the pulmonary artery, distributed through the lungs, from which it is brought back by four principal pulmonary veins, and poured into the left or systemic ventricle, which contracting with great force, propels it into the aorta, it being prevented from regurgitating into the auricle by the action of the mitral valves. The blood being propelled into the aorta, is by its trunks and branches, distributed to every part of the body, and brought back as before by the ramifications and trunks of the vena cava.

199 Circulation II. In the fœtal state, the blood being brought back in the hu- from every part of the body by the ramifications and man fœtus. trunks of the vena cava, is poured into the pulmonic auricle of the heart, where it is mixed with the blood brought from the placenta. A part of this blood is conveyed from the sinus of the auricle while in a state of dilatation, through the oval hole, into the systemic auricle; while another part, by the contraction of the auricle, is thrown into the pulmonic ventricle, which contracting, propels it into the root of the pulmonary artery. From this the greater part of the blood passes through the arterious canal, that in the fœtus joins the pulmonary artery to the aorta, in this latter; while the remainder is distributed by the ramifications of the pulmonary artery to the lungs, and brought back by the pulmonary veins to the systemic auricle of the heart. By the contraction of this auricle, the blood is thrown into the systemic ventricle, which contracting, propels it into the aorta. Now, while one part of the blood is distributed by the numerous ramifications of the aorta to every part of the body

tion.

of the foetus, another part is carried from the internal of Circulailiac arteries, through the two umbilical arteries, into the placenta, from which it is conveyed through the umbilical vein to the sinus of the liver. Hence, one part of the blood, without entering the liver, is transmitted by a branch of the umbilical vein, called venous duct, into the left branch of the vena cava hepatica; thence into the inferior great vena cava ; while another part, by another branch of the umbilical vein, flows into the left branch of the vena porta, by the numerous ramifications of which it is distributed to the liver. From the liver it is carried by the vena cava hepatica into the inferior great vena cava, whence it is conveyed, with the rest of the blood, to the pulmonic auricle of the heart, to be distributed as before.

200

We must now briefly consider the powers by which Powers carthe blood is made to circulate through such a multitude rying on the circulation. of vessels, so infinitely ramified, and differing so much in their diameter. It seems generally allowed at the present day, that these powers are chiefly the immediate muscular action of the heart, the action of the arteries, the valves of the veins, and the pressure produced on some of the veins by the action of the muscles that lie contiguous to them.

201

vol. ii.

1. That the heart must possess very considerable force Action of in propelling the blood through the arteries, may be the heart.. supposed from the great muscularity of its ventricles; and this force has been proved by experiment and observation. From experiments made by Hales, viz. that of inserting a glass tube into a large artery, and measuring the height to which the blood ascends at each pulsation, it has been calculated, that the human carotid artery is capable of projecting its blood to a perpendicu lar height of seven feet and a half; and if we estimate the surface of the systemic ventricle at 15 square inches, we shall find that it sustains a pressure of 1350 cubic inches, equal to 51 pounds weight, which it has to overcome by its contracting force *. This is a moderate * Hales's computation of the force of the heart, for Borelli esti-Statical mates it at 180,000 pounds, while Keill diminishes it Essays, to eight ounces. Senac again, from having observed, that if a weight of 50 pounds be attached to the foot, with the knee of that side placed over the opposite knee, the weight will be raised at each pulsation, and allowing for the distance at which the weight is placed from the centre of motion, computes the force of the heart at 400 pounds + Blumenbach has seen the blood + Traité projected from the carotid of an adult more than five du Cœur. feet On a medium calculation, estimating the quan- Institu tity of blood contained in the body at 30 pounds, the siolog. number of pulsations in each minute at. 75, and the quantity of blood ejected from the systemic ventricle at each contraction at two ounces and a half, we shall find that the whole 30 pounds of blood will be carried through the whole body no less than 23 times in an hour, or the circulation will be completed in less than three minutes. From these circumstances we must infer that the impelling power of the heart is very great, and fully adequate to the office which it has to perform.

Various hypotheses have been formed to explain how the heart and arteries are excited to motion, but our limits will not permit us to detail them. Our readers will find them related at considerable length, and fully examined in the Principes de Physiologie of Dumas, tom.

iii..

tiones Phy

Of Circula- iii. p. 332-364. The general opinion at present entertained on this subject is, that the heart is excited to action by the stimulus of the blood.

tion.

202

Action of

the arteries.

*Hunter on the Blood,

P. 114.

203 Valvular

the veins.

2. Although it is more than probable that the action of the heart is the principal instrument in carrying on the circulation, there can be no doubt that the arteries contribute essentially to this office. They are evidently muscular, and are possessed of considerable irritability; are supplied with numerous nervous filaments, and are nourished by small arterial branches, commonly called vasa vasorum. Nay, we know that they are susceptible of contraction; for when we divide an artery in the living body, the divided extremities gradually contract, till, if the animal is not killed by the experiment, the aperture is at length obliterated *. Lastly, there have been instances of fetuses without a heart; and as we must suppose that, during the life of the fœtus, the circulation was going on, it is a natural inference that this was chiefly effected by the contraction of the arteries, and not entirely by the impelling power of the circulating system of the mother.

3. It cannot be supposed that the veins have any imstructure of mediate action on the blood, as they exhibit no circular fibres like the arteries, except in the immediate vicinity of the heart; but their valvular structure must contribute to the carrying on of the circulation, from the opposition it gives to the return of the blood, so that what is called the vis à tergo, or impelling power from behind, aided by the conical form of the veins, may have its full effect.

204

Action of the muscles.

205 Circulation of the in

mals.

4. That the action of the muscles has considerable influence in propelling the blood towards the heart, in those veins that lie in their neighbourhood, is evident from the effect that bodily exercise produces in accelerating the circulation, and from the efficacy of friction in removing congestions of blood in the veins of the extremities, and in the more familiar instance of promoting the swelling of the veins of the arm by the same means in the operation of bleeding.

The circulation in those animals whose structure apferior ani- proaches to that of man, differs little from what is above described. There are indeed some peculiarities, a few of which we shall presently notice. An account of the circulation in the cetacea will be found in the article CETOLOGY, N° 140-145; that of the reptiles is described under ERPETOLOGY, p. 309.

206

Circulation of the mollusca ;

207 in the vermes;

The mollusca possess an evident and powerful circulation. Most of them have a simple heart, consisting of one auricle and one ventricle; and in these the vena cava performs the office of an artery, carrying the returning blood to the gills, whence it passes to the auricle, and is afterwards thrown into the aorta. There is a peculiarity in the cuttle fish, which has a heart consisting of three ventricles, without any part that can properly be called an auricle. Two of the ventricles are placed at the roots of the two bronchiæ, and have each a branch of the vena cava, by which they receive the blood from the body, and propel it into the bronchiæ. The returning veins open into the middle ventricle, and from this the aorta proceeds.

Some of the vermes, as the leech, and the tribes of ・the naias, nereïs, and aphroditæ, and some species of lumbricus, have no heart, but they have circulating vesin crusta- sels with evident contraction and dilatation.

cea.

208

In the crustacea, the circulation is performed by a

There is no circulation in insects; but these animals Contract. have running along the back a membranous tube, in ing and diwhich alternate contractions and dilatations are percep-lating tube tible. This tube, however, is closed at both ends, and in insecte. has no vessels proceeding from it.

210

From the researches which evince circulation to be a No circ function so general among animals, some are disposed to lation in think it takes place in all living bodies. But notwith-plants, standing the fashionable language of circulating fluids, of veins, arteries, and even of valves, in the vegetable structure; yet nothing performing the office of a heart, and nothing that seems to conduct fluids in a circular course, has been found in plants. In the vegetable kingdom, the chyle is distributed to all the parts, from the numerous vessels which convey the sap; and these vessels, being fitted by their structure to carry the sap either downwards or upwards, from the branches to the roots, or from the roots to the branches, is the reason why plants inverted in the ground will send forth roots from the place of their branches, and send forth branches from the place of their roots. Even a similar distribution of the chyle takes place in some animals. In the human tænia, in the fasciola hepatica (fluke) of sheep, and in most polypes, the chyle, without a circulating system, is conveyed directly to the different parts from the alimentary canal.

For an account of the motion of sap in plants, see Darwin's Phytologia, a paper by Mr Knight in the Philosophical Transactions for 1801, Wildenow's Principles of Botany, sect. 276, and the article PLANT in this Encyclopædia.

211 nor in some

animals

212

and sensa

The relations that subsist between the function of cir- Relations culation, and those which we have already considered, between are very important. We shall begin with those of cir- circulation culation and sensation. That the functions of the ner- tion. vous system must be considerably influenced by the circulation of the blood, may be supposed à priori, from the large quantity of blood sent to the head, this being, on a moderate calculation, about one-tenth of the whole. A certain quantity of blood in the vessels of the brain seems essential to the due performance of the functions of that organ; and those animals, which, like man, have the blood sent in greatest quantity, and with greatest impulse, seem to possess the faculties of the brain in the greatest perfection, while those in whom the motions of the blood towards the head is much retarded, as in the sheep and cow, are remarkable for mildness and stupidity. When, however, the quantity of blond becomes too great, or its impetus too violent, the faculties of the brain are impaired, or altogether destroyed. No man, and very few other animals, can remain suspended with the head downwards for any long time, without dangerous, and commonly fatal consequences. The bat, indeed, is a remarkable exception to this rule, for this animal can hang by its hinder feet for days or weeks together, with perfect safety, a circumstance that may be accounted for from the very small quantity of blood contained in its circulating vessels. Again, the brain exerts an evident influence on the circulation. It is well known how much the action of the heart and arteries is quickened, impeded, or rendered irregular, by

the

tion.

tion.

215

The relations between the circulating and digestive Of Circulaorgans are proved by the sudden acceleration of the pulse from stimuli received into the stomach; from the diminished circulation or sudden cessation of the heart's mo- Relations tion from powerful sedatives received into the same or- of circulagan, especially the prunus lauro-cerasus; from the irre- tion with gularity produced in the circulation in consequence of digestion. dyspepsia, and many other considerations.

Of Circula- the passions of the mind. Cases are recorded in which these passions, carried to excess, have altogether stopped the circulation, and produced instant death. Fainting is often brought on by the sight of a disgusting or terrifying object, or by the odour of perfumes, or of substances to which the person has a particular antipathy. The sympathy between the heart and the nervous system is farther shown by the violent pain below the sternum, and sometimes in the arm, in cases of organic disease of the heart.

and motion

The circulation is even affected by intense thought; and we have heard of a bleeding from the nose being brought on by long and deep study, while the body was 213 in a reclining posture, namely in bed. Relations The functions of circulation and motion are intimatebetween ly related. It is scarcely necessary to notice the accelecirculation ration of the pulse, in consequence of exercise and labour, or to remark that in indolent and sedentary people the circulation is generally slow and languid. In general, too, the blood circulates with most rapidity in those animals who are formed for quick motion, though the instance above quoted, of the bat, shows that the quickness of motion does not depend on the quantity of blood. Several curious anatomical facts have rendered it probable, that the production of quick or slow motion depends in a great measure on the mode in which the arterial branches are distributed to the moving organs. The arterial branches that supply the organs of voluntary motion, are divided in such a manner as to impede the motion of the blood towards these organs as little as possible: their ramifications are therefore few, and they go off from the trunk at very acute angles; whereas those that supply the viscera are at nearly right angles, are often tortuous, and are otherwise so constructed as frequently to impede the flow of blood. Physiologists have even explained the greater power that is generally found in the right arm, and the greater readiness with which most people use that arm, by the manner in which the right subclavian artery comes off from the aorta. This indeed we are disposed to consider as fanciful, and to attribute the more ready use of the right arm solely to habit and early instruction not to employ the left.

slow mov

mals.

214 DistribuIn some animals that are remarkable for slowness of tion of the motion, as the lemur tardigradus, or slow lemur, and arteries in the bradypus tridactylus, or common sloth, there is a the limbs of curious construction of the arteries that are distributed ing ani to the limbs of these slow-moving animals, which must have the effect of breaking the force, and impeding the velocity of the blood towards these limbs. In the lemur tardigradus, a specimen of which was dissected by Mr Carlisle, it was found that the subclavian artery and the external iliac were, soon after rising from the general trunk, divided into a great number of equal-sized cylinders surrounding the principal artery, now diminished to a very small size, and that each of these branches was sent to each of the principal muscles of the limbs, while the other arteries that supplied the other parts of the limbs were divided in the usual arborescent form. Struck with this appearance, Dr Shaw and Mr Carlisle afterwards examined several specimens of the sloth, and found a similar conformation, while in other species of bradypus, not remarkable for slow motion, no such appearance took place*. Fig. 5 & 6. Plate CCCCXVIII. represent the division of the arteries in the slow lemur above described.

Shaw's General Zoology,

vol. i.

VOL. XVI. Part II.

216

That there is an intimate relation between circulation Relation of circulation and absorption, cannot be doubted, though the nature and absorp and effects of this relation are not yet well understood. tion. We know that the vessels sympathize with the absorbents in their activity or languor; that when the absorbents are languid in their action, the blood-vessels, especially the exhalants, are in a feeble or relaxed state, and that the absorbents are often roused to greater action by remedies that first act on the circulation.

217

blood.

Numerous experiments have shown how much the Action of colour and consistence of the blood are altered by the the vessels mere action of the vessels; and this discovery has ena- on the bled us to conjecture with more probability than we did formerly, why in infauts and phlegmatic persons the blood is paler, in the choleric more yellow, and in the sanguine of vermilion red. It explains likewise in some measure, why the blood varies in the same individual not only with regard to the state of health, but likewise at the same instant; and why the blood which circulates through the veins has not the same intensity of colour, nor the same consistence, as that of the arteries; and why the blood which flows through the organs of the breast differs from that which passes languidly through the viscera of the lower belly. This power of the vessels over the blood will bring us also to the true cause why the vessels vary in the density of their coats and in their diameters; why they are sometimes convoluted in a gland, and why they sometimes deposit their contents in a follicle; why they are sometimes of a spiral form; why the branches strike off at various angles; why they are variously anastomosed; why they sometimes carry the blood with dispatch, and sometimes slowly through a thousand windings. By these means their action is va ried, and the blood prepared numerous ways to answer the ends of nutrition and secretion.

On the varieties of the pulse, and the morbid affections of circulation, see MEDICINE, No 96—104.

CHAP. VIII. Of Respiration.

218

WHILE the fluids are passing through the body, by Necessity what is called the greater circulation, they give out cer- of respirătain parts or principles, partly for the purpose of nutri- tion. tion, and partly to free the system from noxious matters. In order to regain some of the principles which they have lost, it is necessary that they should be exposed to the influence of atmospheric air; for which purpose they are made to pass through appropriate organs, which, as we have already observed, are in general either lungs or gilis. The action by which the fluids and the air are made to act on each other, is called respiration, and consists of two kinds, inspiration, by which the air is received into the body, and expiration, by which it is again thrown out.

So essential is respiration to the system, that snails, chameleons, and some other animals, can live for years, without any apparent nourishment, provided they be not 3 R excluded