GO 



ORGANS OF SUPPORT. 



mity, and all the posterior portion is freely moveable, to 

 give impulse to the tail in swimming. In birds, (Fig. 29. 2,) 

 where the head and neck are used as a hand and arm, 

 for all prehensile pur- 

 poses, the fixed thora- 

 cic portion (a,) of the 

 column is placed near to 

 its posterior extremity, 

 and the anterior portion 

 is free for extensive mo- 

 tion. Most quadrupeds 

 (Fig. 29. 1,) and reptiles, 

 balanced on two pairs of 

 extremities, hold an in- 

 termediate place, and have the ribbed and solid portion 

 ( a, ) of their trunk placed near the middle of the 

 column. 



XIX. Pisces. The bones of fishes contain less gelatine, 

 and a larger proportion of water than those of higher 

 classes, and are less dense and compact in their texture. 

 The soft bones of cartilaginous fishes yield more water 

 than those of osseous fishes, and they contain the soluble 

 salts of soda, the chloruret, the sub-carbonate, and the 

 sulphate, while the more dense bones of osseous fishes 

 are strengthened, like those of higher classes, with the 

 more insoluble phosphates. The bones of fishes resemble 

 those of the embryos of higher animals, not only in their 

 soft, gelatinous, or cartilaginous character, but also in 

 the isolated condition of all the elements, or centres of 

 ossification, of the more complicated bones, especially of 

 the head. The skeleton of fishes consists almost entirely 

 of the vertebral column, from the extremity of the face 

 to the end of the tail, like that of the embryos of mam- 

 malia at a corresponding stage of their development. The 

 bodies of the vertebrae are composed of concentric layers, 

 as represented in Fig. 30. 0, which are broadest at the 

 circumference, and become narrower as they approach the 

 centre, where there is commonly a small hole (Fig. 30. b, b.) 

 These bodies are therefore concave both on their anterior 

 and posterior surfaces, and when applied to each other, 

 they leave large intevertebral spaces between them, which 



