THE NERVOUS SYSTEM. 



1U9 



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 (Blindwonns), or the one 

 pair may be present without the other as 

 MIUI 11 rudiments. In most cases, however, 

 both pairs of extremities are completely 

 developed, and provided with live 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-C& 

 cidedly higher than that of the Arityhilid. 

 The hemispheres are distinguished by j^ _j 

 their considerable size, and begin to cover 

 the me>encephalon. The cerebellum shows 

 various grades of development progressing 

 from the Snakes to the Crocodiles, and in 

 tin- 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 FIG _ 632 _ Brain of the Al!i( , (lfoi . 

 the trigeniinal, and the giossopharyngeal seen from above (after Rabl- 



. , -!~. , Ruckhard). T7i. prosencephalon 



appears as an independent nerve, which (Pcrebval hem i sp heres) ; Mh, me- 

 lias. however, several connections with the ^ncephaiou (corpora bigemina); 



, . Cb, cerebellum; Mo, medulla 



vagus. Ihe spinal accessory also arises O biougata ; i, olfactory nerve; 

 independently except in the Snakes. */, optic ; JF, troehiear (fourth); 



1 / I", trigerninus (fifth) ; TT/7, audi- 



Filially the nypOglOSSal, which passes Out tory nerve ; IX, giossopharyngeal 



through a single or double opening in the ( ninth ); x < va ^ s (tenth); xi, 



spinal accessory (eleventh); 1C, 



skull, enters the category of the cranial first spinal nerve; -ic, second 



nerves. spinal nerve. 



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 acconi- 



