Having noticed, as fully as our space permits, the modifications which the typical vertebra undergoes in various animals, and in different parts of the same animal, we now come to the more difficult subject of "the archetype Tertebrate skeleton," which is made •up of a series of vertebra arranged in a continuous row, The accompanying scheme or diagram represents prof. Owen's conception of the common pattern or arche type of the vertebrate skeleton. It is difficult at first sight to see any resemblance between this figure and the human skeleton; but, in fact, the human skeleton, of all others, recedes the furthest from the common pattern; and if we turn to fishes, which were the first form of vertebrate life introduced into this planet, we tihd that they deviate the least from the archetypal idea. If proof be demanded that a given bone in the human skull is an clement of a particular vertebra, it is afforded by tracing the same bone through its various modifications in mammals, birds, reptiles, and fishes, till the simple archetypal form is arrived at. The skull is found to be but a continuation of the backbone, and to consist of four vertebra or segments, corresponding to the four con secutive enlargements of the nervous system, which we call the brain. These segments, reckoning them from behind forwas'd, are termed the occipital, the parietal, the frontal, and the nasal segment. Each segment consists of a neural and a hemal arch.
The neural arches are: N. I. Epencephalic arch (bones Nos. 1, 2, 3, 4 in figure). N. II. 3Iesencephalics arch (bones Nos. 5, 6, 7, 8 in figure). N. III. Prosencephalic arch (bones Nos. 9, 10, 11, 12 in figure). N. lihinencephalic arch (bones 14, 15 in figure).
The hemal arches are: H. I. Scapular arch (Nos. 50-52).. H. II. Ilyoidean arch (Nos. 38-43). H. III. Man dibular arch (Nos. 28-32). H. IV. _Maxillary arch (Nos. 20-22).
The jaws are the modified hemal arches of the first two segments; and the mouth opens at the iuterspace between these arches. The position of the vent varies (in fishes), but always opens behind the pelvic arch, 6 62, 63,. p, where this is ossified. Outlines of the chief ossified developments of the dermo-skeleton, in different vertebrates, are added by prof: Owen to the nenro-skeletal archetype; as, for example, the median- horn, sup ported by the nasal spine, 15, in the rhinoceros; the pair of lateral horns developed from the frontal spine, 11, in most ruminants; the median folds, DI, DTI, above the neural spines, one or more in number, constituting the dorsal fin or fins in fishes and cataceans, and the dorsal hump or humps in the buffaloes and camels; similar folds are 'sometimes developed at the end of the tail, constituting the caudal fin, C, and the anal fin or fins, A, of fishes.
It has been already remarked that bones which diverge as rays are formed from one or more parts of a vertebra. These " diverging appendages" are mainly connected with the hemal arches, and those which especially concern us are the pectoral appendages of the scapular arch, which become developed into fore-limbs or arms, and the pelvic appendages which are attached to their supporting lima] arch, 63, ha. If we examine the skull ofa cod-fish, in which the bones have been arranged according to the segments or vertebra to which they belong, we observe that the occipital vertebra has a widely expanded hemal arch, consisting of three pairs of bones with diverging appendages.
The special names given by Owen to the various elements of that hemal arch, from above downward, are "suprascapular," No. 50; "scapula," No. 51; "corticoid," No. 52. The scapular arch thus formed supports and protects the heart or center of the Kemal system, and in most fishes supports the pectoral fin, while in other animals the appendage that here becomes a fin is modified into a fore-leg, a wing, an arm, and a hand. Some of the special names originally employed in human anatomy are retained and applied to like parts in the pectoral fin of the fish; but it will be observed that prof. Owen designates each hone not only by a name but by a numeral. Of the two fiat bones connecting the -fin with the corticoid, the upper one is the "ulna," No. 54; the lower, the "radius," No. 55. the row of short bones joined with these are the "carpals," No. 56; beyond which are the metacarpals and phalanges. Ascending from fishes to reptiles, we find that, in the lower batrachia (as the a mphiunia), the scapula are detached from the occiput, and that other important modifications-have occurred, The corticoids are well expanded, three segments of the diverging appendage arc ossified, and two of these segments are bifid, showing a simple beginning of the radiating multiplication of parts. The first segment is the seat of these modifications, which have acquired for it the special name of " humerus;" the two divisions of the next segment of the appen dage are called "ulnae and " radius;" the gristly mass is the carpus, and the two bony divisions are the digits or fingers. We have here got so distinct a rudimentary arm, separated from the bead, although, according to the views propounded in this article, an appendage of the occipital segment of the. cranium, that it is unnecessary to trace'the further modifications that ensue. which lead finally to the arm and hand of man. It is only necessary to remark that in mammals, except amongst the non-placental orders, the eoracoid hone is reduced to at mere rudiment, being known as a process of the scap ula, and that its function—namely, that of keeping the shoulders apart—is performed by the clavicle, which, according to Owen, is the hemapophysis (58) of the first cervical vertebra (see fig. 3). With regard to the pelvic arch, awe have only space to add that it must be regarded as the hem] arch of one or more of the pelvic vertebra; and there is undoubted evidence to show that the pelvic and scapular arches are constructed on the same plan; the " ileum" answering to the scapula, the " ischium" to the corticoid, and the "pubis" to the clavicle; and the same remark applies to the pelvic and scapular appendageS.
"Of this," says Mr. Holden, probably our best authority on human osteology, "a student may rest assured that, however minutely he May have scrutinized the bones, he cannot understand them unless he knows something of the ` vertebrate archetype.' Without this knowledge he is like one who speaks a language fluently, but is ignorant of its grammar. The ` archtype' may be said to be the grammar of all osteology."