OSSEOUS SYSTEM. (CoMP. ANAT.) 



823 



1. The spinal column, the centre of the whole 

 fabric enclosing in a canal formed by arches 

 surmounting its dorsal aspect the medulla spi- 

 nal is or the axis of the spinal portion of the 

 nervous system. 



2. The cranium, essentially composed of 

 vertebrae; 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. 



3. Of a frame of bones appended to the 

 anterior part of thecranium for the lodgement of 

 the organs of those senses that are immediately 

 in connection with the encephalon, forming 

 what, taken collectively, is called the face. 



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 

 presenting corresponding parts, to which the 

 names pelvis, thigh, leg, tarsus, metatarsus, 

 and toes are respectively appropriated. 



The most complete skeleton with which we 

 are acquainted among existing Vertebrate 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 di visible i nto 

 a cervical region (fig. 432, a, b) interposed be- 

 tween thecranium and the thorax, although ribs 

 (o) are appended even to the cervical vertebrae. 

 The dorsal region (b, c) supporting the thoracic 

 ribs, the lumbar (c, t/), 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 any thing rather 

 than the human os coccygis ; for here, so far 

 from its being formed merely of the rudiments 

 of the bodies of almost obliterated vertebrae, 

 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 

 numerous than in the skull of our own species, 

 as we shall explain more minutely hereafter : 

 see fig. 441, where they are delineated on an 

 enlarged scale. The thorax consists of dorsal 

 ribs (/) and of sternal ribs(w), which are equally 

 important elements of the skeleton and of the 

 sternum, here situated much as in the human 

 subject. Behind the sternum, moreover, and 

 extending from it quite to the pubic bones, 

 there is in the Crocodile a set of ventral ribs (ft) 

 to which in Man there is nothing analogous, 



except, perhaps, the tendinous intersections still 

 lingering in the recti muscles of the abdomen. 

 The shoulder (/>, </) 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. 



Having premised thus much, we may now, 

 without further preface, plunge more deeply 

 into our subject, and, taking in detail all the 

 elements that are recognised by modern anato- 

 mists as belonging to the osseous system, exa- 

 mine them separately in the various aspects 

 under which they present themselves in the 

 different classes composing the Vertebrate por- 

 tion of animated nature. 



Spinal column. Commencing our analysis of 

 the skeleton by an examination of the spine as 

 being the most essential portion of the osseous 

 system, the primary or central part to which all 

 others that are met with in the different classes 

 of Vertebrate may or may not be superadded in 

 accordance with the conditions under which 

 they are appointed to exist, we shall soon per- 

 ceive that both in texture and composition it 

 offers very important varieties. 'In the Myxine 

 and Lampreys it is a simple stem of extremely 

 soft cartilage, almost gelatinous in its con- 

 sistence, which traverses the axis of the body, 

 presenting, when superficially examined, no 

 appearance of division into separate vertebra; ; 

 and it is not uninteresting to observe how, 

 advancing from this simplest form of spine 

 through various tribes of Fishes, its separation 

 into distinct pieces is gradually effected. But 

 even in the Lamprey, on strict examination, 

 there are perceptible in the arches that embrace 

 the spinal canal and on the surface of the soft 

 cord that represents the bodies of the vertebrae, 

 slight indications of an incipent division into ver- 

 tebral pieces, which are represented by slender 

 rings of ossific matter that encircle at intervals 

 the soft cartilage upon which they sensibly 

 encroach. In a more advanced form of the 

 spine, these ossified rings are considerably 

 increased in their relative proportions, and en- 

 croach further and further upon the cartilaginous 

 stem until they penetrate even to its centre, and 

 are then no longer dubiously the representatives 

 of the bodies of so many vertebrae. In the 

 generality of Fishes, indeed, the central part 

 remains unossified, so that a cartilaginous axis 

 traverses the vertebral column from end to end. 

 At last even this is obliterated, and the vertebral 

 centres are completely formed. 



But even before the bodies of the vertebrae 

 are thus perfected, the lamina 3 destined to 

 enclose and protect the spinal cord are fully 

 formed by the deposition of osseous matter, as 

 may be readily seen in the Sharks and Rays 

 and many other cartilaginous Fishes, in which, 

 although the complete consolidation of the 

 body has not yet been achieved, the spinous 

 and other processes destined to form the fulcra 

 upon which muscular action is to be exerted 

 are so ossified as to afford the needful solidity 

 and strength. In these races of Fishes, indeed, 

 the condition of the spinal column is not a 

 little remarkable, inasmuch as in the Skates the 

 anterior vertebrae are so consolidated by an 



