THE NERVOUS SYSTEM. 



199 



found in the anal region, but they are hidden beneath the skin, 

 except the terminal part, which bears a claw. In the Lacertilia the 

 extremities present very various grades of development ; while the 

 pectoral and pelvic girdles are without exception present, though 

 they are sometimes very rudimentary, the 

 anterior and posterior limbs may be com- 

 pletely absent (Blindworms), or the one 

 pair may be present without the other as 

 small rudiments. In most cases, however, 

 both pairs of extremities are completely 

 developed, and provided with five digits. 

 Sometimes the digits are connected by 

 swimming membranes (Crocodiles), or the 

 extremities are modified to form flat 

 swimming fins (fossil Hydrosaurians and 

 Turtles). 



The nervous system (fig. 632) is de- 

 cidedly higher than that of the Amphibia. 

 The hemispheres are distinguished by 

 their considerable size, and begin to cover 

 the mesencephalon. The cerebellum shows 

 various grades of development progressing 

 from the Snakes to the Crocodiles, and in 

 the latter recalls that of Birds by the 

 contrast of its large median lobe and its 

 small lateral appendages. Of the cranial 

 nerves the facial is no longer united with FlG> 632 _ Brain of the MUgator 

 the trigeminal, and the glossopharyngeal seen from above (after Rabi- 



, .. 1-1 Riickhard). Vh, prosencephalon 



appears as an independent nerve, which (cerebral hemispheres) 



has, however, several connections with the 



vagus. The spinal accessory also arises 



independently except in the Snakes. 



Finally the hypoglossal, which passes out 



through a single or double opening in the 



skull, enters the category of the cranial 



nerves. 



The eyes are without lids in the Snakes, Geckos, and Amphisbsenas, 

 but are protected in these animals by a transparent capsule, which is 

 separated from the cornea by a space filled with lachrymal fluid. In 

 all other cases there is an upper and lower eyelid. An independent 

 nictitating membrane at the inner angle of the eye is always accom- 



Mh, me- 

 sencephalon (corpora bigemina); 

 Cb, cerebellum; Mo, medulla 

 oblongata; I, olfactory nerve; 

 II, optic ; IV, trochlear (fourth); 

 V, trigeminus (fifth) ; nil, audi- 

 tory nerve ; IX, glossopharyngeal 

 (ninth); X, vagus (tenth); XI, 

 spinal accessory (eleventh); 1C, 

 first spinal nerve ; 2C, second 

 spinal nerve. 



