But passing from these general views, for a more complete consideration of which the reader is referred to another article, (Osssous Tis suE,) we proceed to examine more closely the composition and developement of the skeleton, and here we find difficulties to be encountered of no common kind. Did the skeleton invariably consist of the same number of bones, only modi fied in their shape or position according to.the necessities of the different races of Vertebrata, the task of the comparative anatomist would be easy when he came to investigate their analo gies and relations with each other ; but this is far from being the case: the skeleton of the adult animal does not present the same nuinber of pieces as that of the same creature in a less advanced condition, numerous parts, originally distinct, having become fused and consolidated into one; and, on the other hand, the juvenile being differs from the embryo from circumstances precisely the reverse, seeing that the full complement of bones or centres of ossification has not as yet been developed. Now, as in ascending the scale of living beings belonging to the Vertebrate division of the ani mal creation, we find that nature can arrest the further advance of ossification at any assignable point of developernent, leaving some parts per inanently atrophied, while others are allowed to attain their full growth, we find great varieties in the com position of the osseous framework. The Tadpole, were its growth arrested before its limbs begin to sproutovould be a Fish. The Frug,if it ceased to grow when its limbs were but par tially formed, would be a Siren or a Fracas, having little or nothing, in common with the adult creature as regards the configuration or even the number of the bones in its skeleton. Which of die three conditions must the com parative anatomist refer to in order to estimate the condition of the bones composing the frame work of this Batrachian ? The importance of this inquiry will be at once obvious, as, either in the first instance there must have been a much greater number of bones developed than are met with in what is usually considered a complete skeleton, or in the adult animal the bones have become too much confused with each other to allow us at all to estimate their real condition. It is sufficient indeed for any person who is only acquainted with the osteo logy of man, to cast his eyes over the bones entering into the composition of the skeleton of a fish to perceive at once that the nomen clature employed by the human anatomist is by nu means sufficiently ample to afford names to one-half of them, which indeed have no re presentatives in the human body; or even the bare comparison of the adult human cranium with that of the infant of tender age would convince us that in the former there are many more distinct bones than in the latter. The only mode of solving these difficulties is ob viously to study the composition of every part of the skeleton in the most complicated form under which it is met with, and having ascer certained the number and disposition of the pieces of which it then consists, and settled the names and analogies of each, it becomes comparatively easy to point out what parts are deficient in less complex forms of the ske leton.
The number of pieces which can normally enter into the construction of any portion of the osseous appratus having been thus deter mined, these are regarded as the primary ele ments of the skeleton, by the developement, suppression, enlargement, or modified form of which every required variety of the bony framework may be explained, and the con struction of this portion of the animal economy proved to be in accordance with certain immu table laws that may be traced throughout the immense series both of tbe existing and of extinct mces of Vertebrata.
It will readily be perceived after the above remarks that a perfect skeleton, that is, a skele ton presenting all the parts of which it might normally be composed in a complete state of developement, does not exist in nature.
A spinal column may exist alone without either cranium, face, or limbs, as is the case in that strange and rare fish the Amphioxus.' Or
the spine, craniutn, face, and extremities may coexist without ribs or thorax, as in the Frog. The spine and cranium form almost the entire skeleton of many apodal Fishes, while in Ser pents the ribs become the thief instruments employed in locomotion, not even vestiges of legs or arms being visible.
Imag,-ining, however, that a fully formed ske leton, having every apparatus belonging to it, could be pointed out, let us now proceed briefly to glance at the parts of which it would consist, and these we should find to be the following.
1. Tbe spinal column, the centre of the whole fabric enclosing in a canal formed by arches surmounting its dorsal aspect the medulla spi nalis or the axis of the spinal portion of the nervous system.
2. The cranium, essentially composed of vertebra; but here, in consequence of the enlarged size of that part of the cerebro-spinal axis of the nervous apparatus placed within them, exaggerated in size and modified in form.
1 3. Of a frame of bones appended ,to the 1 anterior part of the cranium for the lodgement of 1 the organs of those senses that are immediately I in connection with the encephalon, forming what, taken collectively, is called the J ace.
4. Of a hyo-branchial apparatus forming the framework of the throat, and supporting the organs connected with aquatic respiration. These last of course are only present in animals breathing by gills, and can only be expected to exist in a state of complete developement in the class of Fishes.
5. Of the thoracic apparatus, composed of two sets of ribs—a dorsal and a sternal series- and of the sternum, which itself, when fully developed, is made up of numerous bones.
6. Of a pair of anterior extremities, divisible into shoulder, arm, forearm, carpus, meta carpus, and digits.
7. Of a pair of posterior extremities, con structed after the same model as the last, and presentino corresponding parts, to which the names pelvis, thigh, leg, tarsus, metatarsus, arid toes are respectively appropriated.
The most complete skeleton with which we are acquainted among existing Vertebrata is that of the Crocodiles, the study of which cannot be too strongly recommended 'to the comparative osteologist, as in these creatures all its parts remain permanently in a medium condition, so that the arbitrary divisions of the skeleton adopted by the human anatomist are at once recognisable, although we find others which in Man have no existence. The spine is divisible into a cervical region (fig. 432, a, b) interposed be tween the cranium and the thorax, although ribs (o) are appended even to the cervical vertebrx. The dorsal region (b, c) supporting the thoracic ribs, the lumbar (c, d), the sacrum (e), and the caudal (f) are distinguishable for the same rea sons as in the human subject, notwithstanding that the caudal portion resembles anything rather than the human os coccygis ; for here, so far from its being formed merely of the rudiments of the bodies of almost oblitemted vertebra, I the processes form very powerful levers, and of these there are some developed inferiorly (g) of which no vestiges exist in the human skeleton. The bones of the cranium and face are far more 1 1 numerous than in the skull of our own species, I as we shall explain more minutely hereafter: see fig. 441, where they are delineated on an enlarg,ed scale. The thorax consists of dorsal ribs (/) and of sternal ribs (ni), which are equally important elements of the skeleton and of the i sternum, here situated much as in the human 11 subject. Behind the sternum, moreover, and 1 extending from it quite to the pubic bones, 1 there is in the Crocodile a set of ventral ribs (k) . to which in Alan there is nothing analogous, I I I I except, perhaps, the tendinous intersections still lingering in the recti muscles of the abdomen. The shoulder (p, g) consists, like the pelvis (h, i), of three distinct and important bones, while all the pieces entering into the formation of the extremities very nearly resemble what is met with in the human subject.