1. A state of health, or the tone of the fibre, in which the oxygen exists in its proper quantity.

2. A state of accumulation, in which the fibre is overcharged with the oxygen or irritable principle.

3. A state of exhaustion, in which the fibre is more or less deprived of it.

He likewise arranges the substances, that are capable of coming into contact with the irritable fibre, into three classes.

The first comprehends those substances that have the same degree of affinity for the irritable principle or oxygen, as the organized fibre itself; hence the substances produce no effect upon it.

The second comprehends those substances that have a less degree of affinity for oxygen than the organized fibre has hence these, when they come into contact with it, surcharge it with oxygen, and produce a state of accumulation. They are called negative stimuli.

The third comprehends substances for which oxygen has a greater affinity than it has for the organized fibre. These, therefore, deprive the fibre of its oxygen, and produce a state of exhaustion. They are called positive stimuli.

By way of answer to this fanciful doctrine, we may observe, that if oxygen were so essential to irritability as is supposed in Girtanner's positions, those animals which respire most oxygen should possess most irritability, and those which are capable of living for a long time in deoxygenated air, should have their irritability very low. Now, the reverse of this is found to take place. The muscular fibres of birds which respire more *Johnson's oxygen than most other animals, possess but little irriAnimal tability, while reptiles and worms, which can live for Chemistry, long time without oxygen, are universally and strongly irritable *.

vol. iii.

boldt.

a

127 Hypothesis The other opinion is that of Humboldt, who consiof Hum- ders the galvanic fluid as the source of nervous power, and the primary cause of irritability. He lays down three principles as necessary to excite irritability; viz. 1. Oxygen, which forms combinations with different acidifiable bases. 2. The acidifiable bases (carbone, hydrogen, azote, and phosphorus), of the fibre, with which the oxygen may combine. And, 3. The galvanic fluid.

The galvanic fluid produces, according to Humboldt, the same effect in the animal economy, as the electric fluid in the mixture of azote and oxygen. It is this gal vanic fiuid that, being conveyed by the nerves, brings about the combinations of the oxygen with the different acidifiable bases of the fibres; but when the nerve of a part is tied, it prevents the fluid from passing, which explains the reason of the irritability being destroyed.

The oxygen necessary for these unions is carried by the arterial blood in the course of circulation; and the

acidifiable bases, which are to unite with it, are found to be already present in the fibre.

He found that every thing that augments too much the quantity of the acidifiable bases diminishes the irritability; and that every thing that increases too much the quantity of oxygen, likewise diminishes it; and be thinks it very probable, that the same takes place with respect to the proportion of the galvanic fluid.

Of Irritability.

It is therefore only in a just equilibrium of these principles that the necessary irritability of the parts consists. Upon these principles this philosopher thus explains the production of muscular motion. "In a state of repose, the nerve being inserted in the muscles, the gal vanic fluid is put into equilibrium in organs that touch each other. The spontaneous motion is made by a surcharge of galvanic fluid into the nerve. It appears that the instant we wish to make a motion, the galvanic fluid produced in the brain, is carried en masse towards the part that ought to move, and surcharges the nervous fibres. A discharge from the nerve is then made into the muscles. The particles of these last, animated by increased affinities, approach each other, and Johnson's it is this that constitutes the phenomena of muscular mo- Animal tion *." Chemistry, vol. iii Dumas lays down the following fundamental laws rep. 24. specting animal irritability. 128

1. The essential characteristic of irritability consists Laws of irin a series of contractions and dilatations, determined ritability either by the impression of an external stimulus, or by by Dumas. the simple exertion of the will.

2. Irritability is independent of the action of the nerves; and though generally diffused throughout the animal organization, it belongs rather to the muscular fibre than to any other structure. Its action is in proportion to the number of fibres upon which the irritat ing causes can exert their influence.

3. Irritability is a relative faculty which is not indiscriminately obedient to every species of excitation, but only to those which have some relation to it in the different parts of the living body.

4. There belongs to each organ a specific irritability which requires a peculiar stimulus, accommodated to its nature, and to the kind of functions which it exercises.

5. Irritability has certain vicissitudes of diminution and increase, which vary in the different species of ani mals, in the different organs of the same animals, and under the different circumstances that successively occur in the life of an individual.

6. Irritability is developed with most energy at the moment of death, and immediately after this has taken place.

7. It is multiplied and revived in proportion as the organ which has lost it is divided into a greater number of pieces.

8. It diffuses itself in each part with a velocity proportioned to the activity, number, and duration, of the irritations by which it is excited.

9. There exist mutual relations with respect to influ ence between sensibility and irritability, though each of them is essentially distinct from the other.

10. The exercise of this faculty supposes in the organs

a moderate degree of cohesion, above or below which Principet the action of this force is enfeebled, obstructed, or opposed *."

de Physio

logic, tom.

iii. p. 53

CHAP.

Of Animal Motion.

129 Organs of

motion.

CHAP. IV. Of Animal Motion.

THE organs of motion vary considerably in their nature and connection in the different classes of animals. In some tribes, as in the animalcules and polypes, no distinct organs can be observed. In all above these, however, there are evident muscular fibres, and in many there are hard parts or strong membranes, which serve as points of attachment and fulcra of motion to these fibres. The muscular fibres are to be considered as the essential moving organs, while the parts to which they are attached are merely the passive functions of this organ.

It would be out of place here to enter on a comparative account of the organs of motion; and there is the less occasion for it, as they have been more or less fully described in the former part of the work. The bones, ligaments, muscles, and tendons, with their appendages, as they appear in man, have been amply described in the first and second chapters of the First Part of ANATOMY; and those of other animals have been briefly noticed in the Second Part of that article. Such of our readers as wish for a more particular account, may consult Cuvier's Lectures, vol. i. or Blumenbach's Comparative Anatomy, chap. 1, 2, 3, 4, 5, and 22.

130 Principles Many of the phenomena of muscular motion, as they of muscular take place in man, have also been related under ANAaction. TOMY, N° 85 and 86. We shall here therefore only enumerate and briefly illustrate these phenomena, and shall then proceed to consider a most interesting part of the physiology of motion, the progression of different animals.

Plate

Dr Barclay, in his late excellent work on the muscular motions of the human body, has considered the general subject of muscular action under the following heads, which may be considered as fundamental principles.

1. Fleshy fibres that are continued into tendon by a straight line, shorten the muscle which they compose, in the same degree in which they shorten themselves; those fibres which enter the tendon obliquely, shorten it more, and still more in proportion to their degree of contraction, as they deviate more from the line of the tendon, and approach nearer to the perpendicular, in which last direction they would shorten the muscle most with the least contraction.

This may be illustrated in the following manner. Let AB (fig. 1.) represent a tendon, and CD a fleshy ccccxvII. fibre; and let us suppose that AB is the diameter, and fig. 1. CD the radius of the same circle ADB. It is evident that if the fibre CD should contract so as to bring the point C of the tendon to the point G in the straight line, the extremities of the tendon A, B, (which are supposed to be moveable) would come respectively to E and F; and the situation of the tendon itself would be represented by the angle EGF. If the fibre could be supposed to contract so as to bring the point C to D, the two parts of the tendon CA and CB, would come in contact. If, on the other hand, the fibre CH, which enters the tendon obliquely, were to contract to H, so

as to bring the point C to H, the point A would be Of Animal drawn but a little beyond the middle point C, so that Motion. although this latter fibre is contracted to as great an extent as the former, it has not brought the extremities of the tendon so near together.

2. When two fibres enter a tendon on opposite sides and contract at the same time, they will draw the tendon in the diagonal; and the more nearly the angles which they form with the tendon approach to right angles, the more will the length of the muscle be shortened in proportion to the degree of contraction of the fibres.

Let the fibres BC, BD, BE, BF, BG, (fig. 2.) be Fig. 2: fleshy fibres, inserted into the tendon AB, at the point B, and let us suppose that all these fibres co-operate in bringing the point B to the point G, in the straight line BG. Now the straight fibre BG will be so much shortened when B comes to G, as to be obliterated, while the oblique fibres EB and FB will be shortened only to E a and F b, and the more oblique fibres CB and DB will remain of the length of C c and D d.

3. All muscles that are inserted into bones, are thereby furnished with levers, and as in the action of all levers there are also a fulcrum, a power, and a resistance, these in different cases will be differently situated with respect to one another.

a. In the motions of the head backward and forward on the atlas, the fulcrum is situated between the power and the resistance; or the lever is of what is called in mechanics, the first kind. See MECHANICS, N° 33.

b. When the tibia rests upon the astragalus, and the heel is raised by the muscles of the calf of the leg acting on the tendo achillis, the resistance (which in this case is the pressure of the tibia) is situated between the power and the fulcrum, which are here respectively at the heel and at the toes; or the lever is of the second kind.

c. In raising a weight at the palm of the hand, and bending the arm at the joint of the elbow, the power of action in this joint is situated between the resistance and the fulcrum, which are here respectively at the palm of the hand and the distal extremity of the humerus (D), or the lever is of the third kind.

The shortness of the lever, and the consequently great force of the muscular power required to overcome the resistance in this last case, may be thus illustrated. Let AB (fig. 3.) represent the radius articulated at B with Fig. 3the humerus BC; let DFE represent the biceps flexor muscle running along the humerus, and attached to the radius at E; and suppose a weight W hung to the distal extremity A of the radius. Now, BH will represent the lever of resistance, and BG perpendicular to it the lever of the muscle, which is in this case extremely short.

4. As, other things being equal, all muscles produce a greater extent of motion by a less proportional degree of contraction, and consequently a less proportional change in their fibres, than if they were shorter; those muscles which follow a direct course are seldom attached at the nearest points of the two bones with which

with a less shortening of their fibres, than any straight of Animal muscles attached to the same parallel surfaces.

Let AB and CD (fig. 6. and 7.) be parts of two ribs that are parallel, and that will continue parallel till they are brought in contact by the action of the straight muscles AC, EF, and BD, or by the action of the oblique muscles CE and DE (fig. 7.) and FA and FB (fig. 6.). It is evident, that when the point E comes in contact with F, the length of the straight muscles must be obliterated, while that of the oblique muscles will only be shortened by c E and d E in fig. 7. and ƒA and g B in fig. 6.

Of Animal which they are connected. Hence, beside the advanMotion. tages already mentioned, relations are thus formed between parts at a distance, and the mutual dependence of the functions and their organs is extended and strengthened. On the contrary, those muscles that are not extended along the surface of the bones to which they are attached, are observed to follow an oblique direction, by which they acquire not only contractibility and length, but at the same time a shorter lever than if they had been inserted at the same place with a less obliquity. 5. Of muscles attached to ribs that are parallel, equally moveable, and at right angles to the vertebral column, those that follow a direct course from one to the other, will act on each by equal levers, and make them approach with the same velocity; while those that observe an oblique course will act on each by different levers, and make them approach with different veloci

Fig. 4.

Fig. 5.

ties.

Let AB and CD (fig. 4.) represent two parallel ribs, articulated with the vertebral column at A and C, where they are equally moveable; and let DB and DE be two muscles, the former observing a direct, and the latter an oblique course. The levers of DB will be AB and CD, which, as AC is parallel to BD, are evidently equal; but the levers of DE will be CF and AG, which being of different lengths, the muscle must act with different degrees of force on the different ribs, so that it will make CD, on which it acts with the longest lever, approach AB, faster than it will make this latter approach the former.

Corollary. When bones are not parallel, the muscles that cross in the interval between them, must fall obliquely on both, as it is impossible for a straight line to be at the same time perpendicular to two other lines, unless these be parallel.

6. As all bones move on a centre or axis of motion, while the muscular attachments move in a circum

ference, the muscles, in changing the relative position of any two bones, must, at the same time, be changing the direction of their own action, and varying their le

ver.

Let AB and CD (fig. 6.) represent parts of two parallel ribs, and let AB be moveable on the centre A, and let CF and GE be two muscles inserted obliquely into AB at F and E. Now suppose that by the action of these muscles, AB is brought into the position A b. The points of attachment of the two muscles to AB, will now be fand e, and the muscles will be Cf and Ge, having changed their length, situation, obliquity, and lever.

7. All muscles where the points of attachment move in a circle, draw either towards the centre, or towards the circumference.

8. If any two bones could, by the action of their muscles, be made to approach in a parallel direction, the oblique muscles attached to their parallel and approaching surfaces, would perform a greater extent of motion

9. As, however, no two bones can approach one another in a parallel direction, at least by the action of a single muscle, and as no muscle can continue to act in a direction perpendicular to their two approximating surfaces; a muscle entering them at right angles, when they are parallel, may be placed so near to the centre of motion as to carry the bones through a given space, with a less shortening of fibres than any oblique muscle that has the same origin, but is inserted at a distance, and acts through the medium of a longer lever. Further, a muscle with a less obliquity may be so situated as to carry the bones through a given space, with a less shortening of fibres than any other muscle of the same origin, but of a much greater obliquity.

Motion

Fig. 6. 37.

Let AB and CD (fig. 8.) be two ribs, of which AB Fig. 8. is moveable about the centre A; and suppose that by the shortening of the straight muscle EF, and of the two oblique muscles, EG and EH, AB is brought into the position Ab. The points of attachment, after moving in the segments Ff, Gg, and Hh, will now be respectively at f, g, and h. Now, on the centre E, with the radii Ef, Eg, and Eh, describe three different circular segments. The difference between the present and former lengths of the most oblique muscle EH, will be cH, while the differences between the present and former lengths of the muscles EG and EF, will be only G and F respectively.

n

10. The shortenings which any muscle suffers in carrying round the point of its attachment through a given space, will partly depend on the length of its lever, partly upon its degree of obliquity, partly on its drawing peripherad or centrad, and partly on its acting without or with a pulley (E).

11. The lever of a muscle, which is varied with every degree of obliquity, is also varied by every change in the centre of motion. Where bones are connected by large surfaces, the centre of motion frequently shifts from one part to another; but in general it approaches towards that aspect whither the bone is moving at the time; and as it advances, the muscles recede, to increase their force.

a. The lever of resistance, as well as of the power, is varied by the several changes of position; is sometimes shortened at the time that the lever of the power is lengthened; and vice versa.

If

Of Animal

If AB (g. 9.) represent the radius, BC the humeMotion. rus, DE the biceps flexor muscle, and R the resistance hung to the distal extremity of the radius, it will be evident that, when BA is, by the action of the flexor muscle, brought into the position Ba, the lever of resistance will no longer be BA, but BH, equal to a perpendicular straight line drawn from B, the centre of motion, to the plane of resistance; and, as the lever of resistance has been shortened, the lever of the muscle has been proportionably lengthened. Were the radius to resume its former position, the reverse of these circumstances would take place.

b. Sometimes again, the lever of the power and of the resistance are lengthened or shortened at the same time. Let AB (fig. 10.) represent the tibia, BC the femur, and DEF the crureus muscle; and that the femur, with the weight of the body, is to be raised to the situation Bc; the centre of motion will, during extension, approach towards the muscle at the rotular aspect, while the plane of resistance, as is evident from the figure, will be approaching to the centre of motion.

c. In the changes of attitude, while a bone is turning on its centre of motion, the centre itself is often at the same time describing, either the segment of a circle, or a line composed of circular segments.

Let AB (fig. 11.) represent the foot, BC the tibia, CD the thigh bone, and DE the trunk; and let us suppose that it is required to bring the three last, by the action of their muscles, to the perpendicular BF, so that BC shall occupy the situation of BG, CD the situation of GI, and DE the situation of IF; the point C on the centre B will move in the segment CG, and as C is changing its position in CG, the point D, which moves round the point C as its centre, will, if the extensions be regularly performed in the same time, describe such a curve as DI; for as the point D must necessarily move atlantad and sternad, in order to preserve the centre of gravity, the general direction of its course must be known; and if CG be divided into equal parts, and at each of the divisions a circle described with the radius CD, the points in DI corresponding in number with the points in CG, and at equal distances in the sternal direction, will each be found in the circumference of one of the circles described successively round the point C as it passes along the segment CG.

In like manner, if the extensions of CD and DE be regularly performed in the same time, the point E will describe such a curve as EF, the points in EF being in the circumferences of the several circles successively described round the point D as it moves along the curve DI.

12. When we examine the structure of the animal system, we shall generally find that the motions of the bones, as produced by the muscles, are the combined effects of different forces, and hence that a small number of muscles is enabled to produce, with steadiness * Barclay and accuracy, an almost infinite variety of changes *.

on Muscu tar Motion. Part ii. chap. 3.

137

For more on the general subject of muscular action, and for an account of the principal motions of the human body, we must refer to Dr Barclay's publication.

One of the most interesting enquiries respecting animal motion, is that of the progression of different animals, or of the powers of loco-motion. Progressive Those animals which possess the faculty of changing their place, exercise this faculty by very different orVOL. XVI. Part II.

motions.

+

gans. Some can only creep, as worms, and many mol- of Animal lusca; others can only swim, as all fishes, many of the Motion. mollusca, and some of the testacea. Most birds can both fly, walk, and run, while a few do not possess the power of exercising the first of these motions. All the mammalia, and most reptiles, properly so called, can walk, run, climb, leap, and perform a variety of other motions; and a few of the former class can imitate the flying of birds. We shall briefly examine the mechanism of these different actions, but by way of introduction, we shall first consider how the action of standing is performed.

132

Standing, in most animals, is solely the effect of the Standing continued action of the extensor muscles of all the joints, as is evident from the circumstance, that if an animal, while standing, suddenly dies, or in consequence of some powerful cause, as a strong electric shock, ceases to make the necessary efforts for preserving the upright position, all the articulations of the legs yield to the weight of the body, and bend under it. In some animals, however, the extension of the muscles is so much assisted by powerful ligaments attached to the articulations of the legs, that they are enabled to continue standing for a much longer time, and with much less fatigue than most others. This is the case with birds that perch, and it is particularly remarkable in the stork, which by means of this peculiar mechanism is able to stand on one foot for several days together.

133

The action of standing is somewhat different, according as the animal stands on two feet or on four. That a body may be supported in a vertical position, Standing it is necessary that it be so disposed as to be in a state on two feet. of equilibrium, or that it be so balanced that a perpendicular line from the centre of gravity shall fall within its base. See MECHANICS, N° 193, et seq. It is evident that the more extensive the base is on which the body stands, the less is the danger of its losing its balance. Man can very easily preserve himself in the vertical position, from the broad basis formed by his feet, and from the great power he possesses of separating these to a considerable distance. This latter depends chiefly on the greater weight of his pelvis, and the length and obliquity of the neck of the thigh bone, by which this bone is carried more outward, and removed farther in its articulation, than in any other animal. In man, too, the foot is peculiarly adapted to stand firmly on the ground, from the flatness of its inferior surface, and from having the heel bone so formed as to come in perfect contact with the ground. The muscles that move the foot are also very advantageously inserted, and the extensor muscles of the heel are proportionably thicker than in most of the mammalia.

The thigh of man, when in the erect posture, is in a straight line with the trunk and the leg, whereas in quadrupeds, it is situated close upon the flank, and forms an acute angle with the spine. On this account, the thigh bone of quadrupeds is flat, and proportionally weaker than that of man. The extensor muscles of the thigh are proportionally stronger in man than in the other animals; and as the thigh bone moves upon the pelvis in every direction, these extensors are in man so considerable, that he is the only animal that possesses what are properly called bips.

In consequence of this structure, the human sacral extremities are furnished with a sufficient base, and form very

31

3 P

Motion.

up on the approach of danger, as the hedgehog, the Of Animal armadillos, and the pangolins.

Of Animal very solid bodies for supporting the trunk. Man also possesses several advantages for maintaining the general equilibrium of the body, especially the facility with which he holds his head in the erect posture, owing to the position of the occipital bone, and the horizontal direction of the eyes and mouth. See the article Man, ΝΟ 5. and 6.

134 Standing on four feet.

The quadrupeds that sometimes try to stand on their hind feet only in order that they may either employ their fore feet in taking hold of some object, or avoid keeping their head too low, seem rather to sit than to stand. Their trunk rests at the same time on their hind feet, as far as the heel, and on the buttocks; it is still necessary, however, that their head and neck should be proportionally small, as in monkeys, squirrels, oppossums, c. otherwise the weight of those parts would be too great for the force employed in their elevation; but even when seated, the animal is generally obliged to rest on the fore feet, as may be observed in dogs, cats, &c.

Some quadrupeds use their tail as a third foot, to enlarge the base of the body and when it is strong, it is capable of contributing to their support for some time. We find examples of this in the kangaroos and jerboas. We have already noticed the mechanism in the feet of birds, which enables these animals to support themselves on two legs, though they do not stand in a vertical position, and though the atlantal part of their bodies is advanced more beyond the centre of gravity than the gicral part. Other advantages possessed by birds in this respect are, the great flection of the thigh bone and tarsus; the length of the anterior toes, and the length and flexibility of the neck.

An animal which stands on four feet is supported on a very considerable base; but from the great weight of the head and neck in these animals, their centre of gravity is nearer to the atlantal than to the sacral extremities (F). It is evident from this, that in quadrupeds, the former must sustain almost the whole weight of the body; and we find, accordingly, that they are furnished with very strong muscles. In short, all that the sacral extremities seem to want in muscular force, appears to be transferred to the atlantal.

As in most quadrupeds the head inclines towards the horizon, and the neck is often very long, very powerful means are required to sustain the former. These means are furnished by the great size, and extensive attachments of the muscles of the neck, and especially in many quadrupeds by the cervical ligament. In the mole, which employs its head to raise considerable burdens of earth, the cervical muscles are peculiarly strong, and the ligament is converted into bone.

The body of a quadruped hangs between the four legs, and by its weight tends to draw the spine downwards. This is counteracted by the abdominal musales, especially by the straight muscles, which produce a curvature in the opposite direction. The abdominal muscles act with peculiar force in arching the spine upwards in those mammalia that are covered with scales or spines, and are accustomed to roll themselves

Oviparous quadrupeds or reptiles, have their thighs directed outward, and the inflections of the limbs take place in planes that are perpendicular to the spine. In these, therefore, the weight of the body must act with a much longer lever, in opposing the extension of the knee-joints; and accordingly they have the knees always bent, and the belly dragging on the ground between their legs, whence the name of reptiles.

Motion

135

136

two feet.

In walking on a fixed surface, the centre of gravity Walking. is alternately moved by one part of the extremities, and sustained by the other, the body never being at any time completely suspended over the ground. Animals which can stand erect on two legs, such as Walking on man and birds, walk also on two legs. But several quadrupeds that cannot stand on two feet but with great difficulty, may yet move in that posture for some time with sufficient ease. This arises from its being in general less painful to walk than to stand, the same muscles not being continued so long in action. And also it is less difficult to correct the unsteady motions by contrary and alternate vacillations (a thing easy in walking), than it is to prevent them altogether.

When man intends to walk on even ground, be first advances one foot; his body then rests equally on both legs, the advanced leg making an obtuse angle with the tarsus, and the other an acute one. The ground not yielding to the point of the foot, the heel and the rest of the leg must of necessity be raised, otherwise the heel could not be extended. The pelvis and trunk are consequently thrown upward, forward, and somewhat in a lateral direction. In this manner they move round the fixed foot as a centre, with a radius consisting of a leg belonging to that foot, which, during this operation, continually diminishes the angle formed with the

tarsus.

The leg which communicated this impulse is then thrown forward, and rests its foot upon the ground; while the other, which now forms an acute angle with its foot, has the heel extended in its turn, and in like manner makes the pelvis and trunk turn round upon the former leg.

As each leg supports the body in its turn, as in standing on one foot, the extensor muscles of the thigh and knee are brought into action, to prevent these articulations from yielding; and the flexors act immediately after, when the leg having thrown the weight of the body on its fellow must be raised before it can again be carried forward. As the undulatory motion that necessarily attends a man's walking, cannot be perfectly regulated on both sides, he cannot walk in a perfect straight line, nor can he walk in a direct course with bis eyes shut.

In walking down an inclined plane, or descending a staircase, as the advanced leg is placed lower than that which remains behind, the extensors of the leg must act more powerfully to prevent the body from falling backwards. Again, on ascending such situations it is requisite at each step, not only to transport the body hori

zontally,