VERTEBRATA. 15 



of the posterior portion for the purposes of swimming. In birds, 

 where the neck is used as a prehensile oro-an, the thoracic portion of 

 the column is situate near its posterior extremity. Whilst in quad- 

 rupeds aud reptiles balanced on two pairs of extremities, the solid 

 portion of their trunk is placed near the middle of their column. 



PISCES. 



The bones of fishes closely resemble those of the higher grades 

 of organisation in their embryotic state, not only in their soft carti- 

 laginous character, but also in the isolated condition of their several 

 centres of ossification, especially in the complicated bones of the 

 head. The bones of the cartilaginous tribes are composed of water, 

 gelatine, and the sulphate, subcarbonate and chloruret of soda; 

 whilst the more dense bones of the osseous fishes are indurated and 

 strengthened by the more insoluble phosphates. As the human 

 embryo originally consists almost exclusively of the vertebral 

 column, so also in fishes we find that the spine, and the head, which 

 is only an increased development of it, constitute the most impor- 

 tant parts of the skeleton. 



Spine. — The vertebrae in this class are very numerous, and may 

 be divided into the abdominal and caudal. The bodies of the verte- 

 bras are the elements first developed ; they are the most important, and 

 form almost the entire of the skeleton in the lowest species of cartila- 

 ginous fishes. They are concave on both surfaces ; consequently 

 enclose large spaces, filled with a thin gelatinous fluid ; and in many 

 of the cartilaginous species, the inter- vertebral substances communi- 

 cate and form a continuous elastic chord passing through the entire 

 column as in the lamprey. The spinous processes of the abdominal 

 vertebras are very long superiorly, and assist, by their shanks, to 

 form the spinal canal ; whilst the caudal vertebrae are distinguished 

 by having long spinal processes both above and below. Between 

 the roots of the inferior spinous processes there is enclosed a simi- 

 lar canal, but larger for the passage of the great systemic artery. 



The vertebral column in fishes is constructed in such a manner 

 as to give considerable perpendicular extent to the trunk, and thus 

 favour their horizontal mode of progression. The number of ver- 

 tebra varies greatly; thus in the carp we find 41, in the burbot 57, 

 in the eel 115. and in the shark as many as 207. In the osseous 

 fishes, the ribs form an upper and a lower range ; the latter are 

 better developed and more uniform in their existence than the former. 

 The number of ribs is generally determined by that of the abdo- 

 minal vertebrae. In some, however, as the chaetodon, scomber, &c, 

 the caudal vertebrae are furnished with ribs. These bones articulate 

 with one vertebra only, as a consequence of which they enjoy but 

 little motion, and are but slightly subservient to respiration. In the 

 shark they are cartilaginous ; in the carp they are long and firm ; 

 in the eel short and slender ; and in many genera, as the raia, fis- 

 tularia, &c., they are almost wholly absent. The sternum, when 



