30 ORGANIZATIONOFREPTILES. 



The spino-cerebral axis, or central part of the system, is only found in the verte- 

 brated animals, and is the seat of all sensation, the centre of organic as well as 

 of animal life; as it exercises an influence through the sympathetic nerve over the 

 organs of vegetative hfe, as respiration, circulation, secretion, &ic. The spinal 

 marrow is placed in the canal of the vertebral column, and the brain is the portion 

 situated within the cavity of the cranium. 



The spinal marrow is a cylindrical chord, with a deep furrow on the anterior 

 surface, and a more superficial one on the posterior; it is composed of two sub- 

 stances, belonging to the nervous system in general; a gray or pulpy, and a white 

 or fibrous substance, made up of minute filaments. Its extent varies in the different 

 Reptiles — long in the Sauria, Serpents and Salamanders, extending as far as the 

 caudal vertebrae; in Frogs and Toads, it is short and thick, and ends at the sacrum.* 



The brain, or nervous mass contained within the cranium, may be regarded as 

 the anterior extremity of the spinal marrow, enlarged in size by an increased 

 developement of nervous matter. In all Reptiles it is remarkably small, not filling 

 up even the small cavity of the cranium; its size is diminished not only when 

 compared to that of the Mammalia and Birds, but in proportion to the spinal 

 mari'ow, which preponderates greatly in volume; it differs also from the brain of 

 the higher classes in the form, size, and relative position of its various parts, and 

 in the entire absence of convolutions. Cuviert observes that "the brain of Reptiles 

 may be distinguished from that of all other animals by the position of the optic 

 beds behind the hemispheres." It wants, moreover, a corpus callosum, a fornix 

 with its appendages, and a pons varolii; all the parts of the brain too, are placed 

 one behind the other, instead of being situated one above the other. This different 

 disposition of the different parts depends no doubt on their degree of developement; 

 for were the hemispheres larger, they would necessarily cover the optic beds. 



* Serres, Anat. Comp. du Cerveau, torn. ii. p. 117, observes the spinal marrow exists in the 

 tail of the Tadpole, but disappears when the animal undergoes its metamorphosis. 

 tLegons d'Anat. Comp., torn. ii. p. 174. 



