(1629). In order to simplify as much as possible this important subject, we will select, first, what is generally considered as a single bone - one of the most complex vertebrae of a Fish for instance - and examine its real composition.

(1630). This bone (fig. 306, a) is found to consist of a central portion (a), and of sundry processes derived therefrom, some of which the younger student of human anatomy would at once be able to call by their appropriate names. To the body of the bone (a) he finds appended the arch (b) which encloses the spinal cord, surmounted by its spinous process (c), and with equal facility he recognizes in the lateral processes (d d) the analogues of the transverse processes of the human spine; but here his knowledge fails him, inasmuch as he finds another arch (e) formed beneath the body of the bone, and moreover an inferior spinous process (g), neither of which have any representatives in the human body.

(1631). It is evident, therefore, that the human vertebrae are imperfectly-developed bones, and do not possess all the parts or elements met with in the corresponding portion of the skeleton of a Fish.

Elements of a vertebra.

Fig. 306. Elements of a vertebra.

(1632). The question, therefore, to be solved is this - how many-elements exist in the most perfect vertebra known? and this being once satisfactorily settled, it is easy to detect the deficiencies of such as are less completely developed.

(1633). Taking the example above given as a specimen of a fully-formed vertebra, it has been found to be divisible into the following pieces, all or only a part of which may be present in other vertebrae even belonging to the same skeleton; and these parts are represented detached from each other in the diagram which accompanies the figure (fig. 306, b.) They are, 1st, the centre or body of the bone; 2ndly, two elements (b b), which embrace the spinal marrow; 3rdly, the superior process (c); 4thly, the two transverse processes (d); 5thly, two elements forming the inferior arch, and enclosing the principal blood-vessels (e); and 6thly, an inferior spinous process (g).

(1634). With this key before us, we are able with the utmost ease to comprehend the structure of any form of vertebra that may offer itself. Thus, in different regions of the back of the same Fish, the composition of the vertebrae is totally different: near the tail the vertebrae consist of the body (a), the superior arch (b) and spinous process (c), and the inferior arch (e) and spinous process (g.) In the neighbourhood of the head, however, neither the inferior arch nor spinous process are at all developed; but the transverse processes, which were deficient in the former case, are here of great size and strength. It is obvious, therefore, that the form of a vertebra may be modified to any extent by the simple arrest of the development of certain elements and the disproportionate expansion of others, until at length it becomes scarcely recognizable as constituting the same piece of the skeleton.

(1635). Who would be prepared to expect, for example, that the occipital bone of the human head was merely a modification of a few of the elements of the Fish's vertebra above described enormously expanded, in order to become adapted to altered circumstances? And yet, how simple is the transition! By removing the inferior arch (e) and spinous process (g), and slightly reducing the proportionate length of the transverse processes (d), we arrive at the form of a human vertebra, which exhibits precisely similar elements. Enlarge the arches (b b) that surround the spinal axis of the nervous system, increase the size of the superior spinous element (c), and we have the occipital bone of a Fish; and from hence, through a few intermediate links, we arrive almost imperceptibly at the occipital bone of the human cranium, - the main differences being that the body is in Man divided into two lateral halves, while the superior arches (b) become spread out so as adequately to defend the prodigiously-developed masses of the brain, to which in the human body they correspond.

(1636). One other illustration of this interesting subject. What bones compose a completely-formed thorax? In Man we find, as every tyro knows, 1st, the dorsal vertebrae; 2ndly, the ribs, with their cartilages; and 3rdly, the sternum. But it is not in Man that we must expect a perfectly-developed thoracic framework; it is in the Birds, which are destined to rise in the air by the assistance of their proportionately-powerful thoracic extremities. If, therefore, we examine the thorax of a Bird, we find it composed of pieces which in Man are absolutely wanting: we see, 1st, the vertebrae; 2ndly, the dorsal ribs, firmly articulated on each side both with their bodies and transverse processes; 3rdly, the sternal ribs, extending from the ribs last mentioned to the sternum; and lastly, the sternum, composed, as we shall afterwards see, of various elements not found in the human body. If we prosecute our survey a little further, we shall find this portion of the skeleton offering the greatest possible variety as regards the presence or absence of the elements above enumerated: thus, in the Frog we have vertebrae and sternum, but no ribs; in the Serpent, vertebrae and dorsal ribs, but no sternum or sternal ribs; in Man the sternal ribs are represented by the costal cartilages; and thus a thorax of every required description is constructed by adding or taking away, expanding or contracting certain elements, all of which a typical skeleton might be supposed to contain developed in a medium condition.

(1637). Comparison of the skeleton of a Fish with those of the higher animals demonstrates that the natural arrangement of the parts of the endoskeleton is in a series of segments succeeding each other in the axis of the body. These segments are not, indeed, composed of the same number of bones in any class, or throughout any individual animal; but certain parts of each segment do maintain such constancy in their existence, relation, position, and offices, as to enforce the conviction that they are homologous parts, both in the constituent series of the same individual skeleton, and throughout the series of vertebrate animals. Each of these primary segments of the skeleton is designated a "vertebra," but with as little reference to the primary signification of the word as when the comparative anatomist speaks of a sacral vertebra. A vertebra is defined by Professor Owen as "one of those segments of the endoskeleton which constitute the axis of the body and the protecting canals of the nervous and vascular trunks;" such a segment may also support diverging appendages.