176 CLASS REPTILIA. 



Other vertebrated animals, is formed by a vascular net-work, 

 very complicated and delicate. 



The cerebral hemispheres are placed in front of the optic 

 beds, and do not cover them. Their existence is evident, and 

 they cannot be confounded with any other portion of the 

 encephalic mass, which is by no means the case with all the 

 fishes. Their most usual form is nearly that of a triangle with 

 the base turned behind. As they are more perfect than 

 those of fishes, so are they less variable in their forms through 

 the different species. In the cameleon they form an oval, 

 and are fixed by their base to the cerebral pedicles. Their 

 interior is, as usual, hollowed by a ventricle in which is an 

 hemispheric tubercle, one portion of which represents the 

 corpus striatum. 



As to the organs of sensation which are special in the 

 saurians, we may remark that they all have two eyes placed 

 to the right and left of the head, and tolerably large and pro- 

 jecting in proportion to the volume of the body. These eyes 

 are mobile and lodged in orbits. They are always furnished 

 with lids which vary in number, in figure, and in the degree 

 of mobility. In the crocodiles, for instance, we find three 

 eyelids, two horizontal, and one vertical. The first two close 

 exactly, and have a swelling at their edge, but no eye-lash. 

 The third, semi-transparent, moves from front to back, and 

 is capable of covering the entire eye. It has but a single 

 muscle, which represents the pyramidal muscle in birds, and 

 which fixed in the same manner to the posterior part of the 

 orbit towards the bottom, turns round the optic nerve, re- 

 passes under the eye, and sends its tendon into this lid. The 

 common lizards have instead of eylids, a kind of circular veil, 

 extended in front of the orbit, and pierced with an horizontal 

 cleft closed by a sphincter, and dilated by a levator and a 

 depressor muscle. The lower part of this veil has a carti- 

 laginous disk, smooth, and round, like that of birds. There 



