26 



ANATOMY OF VERTEBRATES. 



the entire animal in the zoological scale. The capsule of the eye- 

 ball, for example, in man, is a fibrous membrane ; in the Turtle, it 

 is gristle ; in the Tunny, bone. The skeletal framework of the 

 Lancelet (Branchiostoma) does not pass beyond the fibrous stage 

 of tissue-change. In the Sturgeon and Skate it stops at the gristly 

 stage, and hence these fishes are called ' cartilaginous.' In most 

 fishes, and all air-breathing vertebrates, it proceeds to the bony 

 stage, with the subsequent modifications and developements above 

 recited. 



13. Classes of bone.- -Bony matter is variously disposed in 

 the bodies of vertebrate animals. The Trunk-fish, fig. 16, dn, dp, 

 dh, the Crocodile, and the Armadillo are instances of its accumu- 

 lation upon or near the surface of the body ; and hence the ball- 

 proof character of the skin of the largest of these mailed examples. 

 The most constant position of bone is around the central masses 

 of the nervous, ib. n, and vascular, ib. h, pi, systems, with rays 

 thence extending into the middle of the chief muscular masses, 

 formmo* the bases of the limbs. Portions of bone are also deve- 



o 



loped to protect and otherwise subser , j the organs of the senses, 

 and in some species are found encasing mucus-ducts, and buried 

 in the substance of certain viscera as, e. g. the heart in the 

 Bullock and some other large quadrupeds. Strong membranes, 

 called ' aponeurotic,' and certain tendons, become bony in some 

 animals; as, e. g. the ( tentorium' in the Cat, the temporal fascia 

 in the Turtle, the ( leaders ' of the leg-muscles in the Turkey, the 

 nuchal ligament in the Mole, and certain tendons of the abdominal 



O ' 



muscles of the Kangaroo, which, so ossified, 

 are called the ( marsupial-bones.' The pri- 

 mary classification of the parts of the osseous 

 system is, therefore, according to their pre- 

 valent position, as in the cases above cited. 

 The superficial or skin-bones constitute the 

 ( dermo-skeleton' ; * the deep-seated bones, in 

 relation to the nervous axis and locomotion, 

 form the ( neuro-skeleton' ; 2 the bones con- 

 nected with the sense-oro-ans and viscera 



o 



form the ' splanchno-skeleton'; 3 those de- 

 veloped in tendons, ligaments, and aponeuroses, the ( sclero- 

 skeleton.' 4 



1 Gr. derma, skin, and skeleton. 



2 Gr. neuron, nerve, and skeleton. 



3 Gr. splaychnon, viscus, or inward part, and skeleton. 



4 Gr. scleros, hard, and skeleton. 



Segment of neuro- and derrao- 

 skcletons, Ostracion 



