350 



ZOOLOGY 



SECT. 



B.ot. 



a nictitating membrane. The greater number of the Geckos and all 

 the Snakes constitute exceptions, movable eyelids being absent 

 in both of these groups ; in the former the integument passes 

 uninterruptedly over the cornea with a transparent spot for the 

 admission of the light ; in the Snakes there is a similar modification, 



but the study of development shows 

 that the transparent area is derived 

 from the nictitating membrane 

 which becomes drawn over the 

 cornea and permanently fixed. In 

 the Chamaeleons there is a single 

 circular eyelid with a central 

 aperture. 



The middle ear is absent in the 

 Snakes, though a columella auris 

 is present, embedded in muscular 

 and fibrous tissue and attached 

 externally, in some cases at least, 

 to the middle of the quadrate. 



Developed in close relation to 

 the epiphysis there is in many 

 Lizards (Lacerta, Varanus, Anguis, 

 Amphibolurus and others), and in 

 Sphenodon, a remarkable eye-like 

 organ the parietal organ or pineal 

 eye (Fig. 1016), which is situated 

 in the parietal foramen of the 

 cranial roof immediately under 

 the integument, and covered over 

 by a specially modified, transparent 

 scale. The pineal eye is developed 

 from a hollow outgrowth of the 

 roof of the diencephalon in front of 

 the epiphysis ; the distal end of 

 this becomes constricted off as a 

 hollow sphere, while the remainder 

 is converted into a nerve. The 

 wall of the hollow sphere becomes 

 divergently modified on opposite 

 sides ; the distal side gives rise to 

 a lens-like thickening (I), the 

 proximal forms a membrane several layers in thickness the 

 retina (r.) : the whole is enclosed in a capsule of connective-tissue 

 (k). The nerve usually degenerates before the animal reaches 

 maturity, so that the organ would appear though evidently, from 

 its structure, an organ of sight to have now entirely or nearly 

 lost its function. 



Met 



FIG. 1015. Brain of Alligator, from 

 above. B. ol. olfactory bulb ; O. p. epi- 

 physis ; HH, cerebellum ; Mcd, spinal 

 cord ; MH, optic lobes ; NH, medulla 

 oblougata ; VH, cerebral hemispheres ; 

 / XI, cerebral nerves ; 1, 2, first and 

 second spinal nerves. (From Wieders- 

 heim's Comparative Anatomy.) 



