example of such appendage is very evident on examining the skeleton of a bird. Attached to its ribs or pleurapophyses, there are seen short flat pieces, which, being directed backwards, overlap the external surface of the next rib behind. (See a, Fig. 37; also a and 65, Fig. 35.) Similar appendages are found, less perfectly developed, in certain reptiles. They also occur in the abdominal parts of the most bony fishes, in which their length is such that they reach even to the skin. They are considered as parts of the primitive segment or vertebra, though less constant than the arches which support them. Now, the simplest form of limb is, in its nature, but very little removed from such diverging appendage; in some of the lower vertebrata, as Protopterus, the limbs are reduced to an unbranched ray.

various

Through species of Amphiuma, and in Proteus, we

observe greater complexity, (though still of low type compared with the extremities of man,) and this goes on step by step in dif

FIG. 87.*

52

51

ferent animals, till we reach the arrangements which characterize the higher forms. The Protopterus, whose simple limbs afford proof of their identity with the diverging appendages of the typical vertebra, present also proofs that the fore and hind limbs are homotypes, both being in that animal precisely of the same simple nature. But even in the higher animals, man

FIG. 37. Occipital vertebra of Protopterus. The hæmal arch is large, consisting of pl. pleurapophysis; h. hæmapophysis; hæmal spine is wanting. The long, simple, jointed ray, a, 57, is the diverging appendage or rudimentary limb.

for example, the resemblance is sufficiently obvious; the arm and thigh, fore-arm and leg, wrist and anklejoint, hand and foot, are the corresponding parts of each limb; these members are therefore homotypes. But under whatever forms the limbs exist, they are supported by inverted arches, the presence of which is more constant than that of the appendages which they support, and for an obvious reason-the arch is required to protect certain important organs which are always present, as the brain and spinal cord, heart and lungs; the appendage of the arch comes in as a secondary instrument, necessary, doubtless, in the economy of the animal; but yet less important in a general sense than the other organs just mentioned.

The parts usually considered as entering into the formation of the upper and lower limbs in man, are the following:-The scapula, or shoulder-blade, and the attached process called coracoid, represent respectively pleurapophysis and hæmapophysis of the occipital vertebra; the clavicles, or collar-bones, are the hæmapophyses of the atlas, or first vertebra of the neck; there is here, therefore transference of arches (which are also imperfect) from their natural position ;-the end of this we shall afterwards examine. Then follows the arm-bone, next the two bones of the fore-arm, called radius and ulna; then the carpus, or wrist, composed of eight bones apparently, but really of ten in two rows; connected to certain of these, we observe five bones of the hand called metacarpus, then follow those of the fingers, styled phalanges, each digit having three, excepting the thumb which has two.

The pelvic portion of the skeleton has been already noticed; it is in like manner an arch supporting diverging appendages, the lower limbs. Each of these consists,

first, of thigh-bone, succeeded by the leg-bones, called tibia and fibula; then follows those of the ankle, the tarsus of anatomists, consisting apparently of seven bones in two rows, which, however, really represent ten primitively distinct pieces. Then follow five metatarsals, or bones of the foot, and connected with their lower ends are the toes, each, with the exception of the great toe having three bones.

Now, whatever be the functions of the extremities in any of the higher vertebrata, we find all, whether fore or hind limbs, constructed on the same plan as that just described, five being the typical number of digits. It may be remarked how different is the relative development of the digits, of thumb, index, middle, ring, and little fingers, styled, respectively, 1st, 2d, 3d, 4th, and 5th in the human hand. The first digit has only two joints; the fifth has the usual number, viz., three, but the whole being short; the second comes next in length, then the fourth; and the third is the most highly developed of all. These peculiarities have distinct reference to the general permanence of these digits respectively, and throw light on certain modifications observed in animals lower in the scale.

In the typical limb, the shortening of the thumb and little finger, or the first and fifth digits, is a step towards their disappearance,† the 2d, 3d, and 4th being more permanent; the two last reaching the ground in the ox, and the longest of the two, namely, the 3d, is the only one which serves as a point of support in the horse. Professor Owen remarks, that "a perfect and beautiful

The same numbers are used to represent the toes; great toe, number 1; little toe number 5.

↑ A similer law reigns in certain plants. In Cruciferæ, (cabbage tribe,) the stamens are usually six, four of these being longer than the other two. In Cardamine hirsuta there are usually only four, the two shorter being absent.

parallelism reigns in the order in which the toes successively disappear in the hind-foot with that of the forefoot."*

Commencing with man as possessing the typical number, and descending to the lower animals, we find that that digit, (the first, or thumb, viz.,) whose uses, par excellence, characterize him, is one of the first which disappears. Departure from the typical five is a characteristic of mammalia lower in the scale, hence the tetra-, tri-, di-, and mono-dactyle limbs common among them.† Descending lower in the scale to fishes, we find the limbs presenting often (with a nearer approach to the simpler diverging appendages) a less subordination to the typical number, there being usually an excess. This, however, as Professor Owen remarks respecting the pectoral fin of the skate and its numerous digits, is not an example of complex deviation, "true complexity not being shewn in the number, but in the variety and co-ordination of the parts." In a word, all diverging appendages or limbs are constructed on a common plan; we shall afterwards examine their numerously diversified modifications for special ends. We also observe in them evident traces of order as regards a law of number, and a general rule in accordance with which they are present or absent, as the necessities of the animal require them or not.

SECT. II.-SPECIAL ADAPTATIONS IN THE STRUCTURE OF

THE SKELETON.

The subject here opened to us is of vast extent, and even not yet thoroughly exhausted by all that has been done in human and comparative anatomy. It must be

* On Limbs, p. 23.

+ This has reference to digits which attain functional size.

acknowledged that the relation between special modifications or departures from the general plan, and final ends of such, have not been determined as to every part of the animal frame. Nevertheless, so many striking examples present themselves to the careful and unprejudiced observer, that it may be considered a legitimate conclusion that there is such a general relation, although the cautious reasoner may hesitate to give a positive decision in every instance which may come under his notice.

We can indicate only some of the more obvious cases illustrative of the coincidence between the principle of order and that of special adaptation. We may appropriately open this part of our subject by glancing at the modifications observed in the vertebrate series in man.

In the cranial vertebræ we observe two remarkable. contrasts in the development of the neural arches; which are more or less extended according to the purpose which they serve in reference to the particular part of the brain over which they are situated. The great size of the nervous centre, that is, the brain, requires a corresponding enlargement in certain neural arches, and this is found to be actually provided. Each vertebra gives protection to corresponding parts of the nervous matter; thus, the cerebellum is protected by the occipital, the mesencephalon (or middle portion) by the parietal, and the prosencephalon (fore-part of cerebral mass) by the frontal vertebra. In all of these the neural arch is ample, in distinct relation to the size of the part requiring defence. The less development, or rather nearly complete obliteration, of the neural arch in the first or nasal vertebra, is commensurate in man (and other ani

* Comprehending also Pons Varolii, Corpora quadrigemina, pituitary body, and third

ventricle.