NERVOUS SYSTEM AND GENERAL SENSATION. 



45 



a* a**). The optic nerves (fig. 22, 2) decussate in most fishes 

 like two fingers laid crosswise ; in the skate the right nerve 

 goes through a fissure in the left ; in bony fishes the nerves 

 cross without any organic intermixture. 



[ 93. In the AMPHIBIA, as the frog and newt, the brain 

 exhibits many of the essential features of the fishes type. In 

 front of the medulla oblongata we observe the small single- 

 lobed cerebellum, c ; before it lies the optic lobes, b, and pineal 

 gland ; and before these are the hemispheres, a, more developed 

 than hi fishes. 



[ 94. In SCALY Fig. 24. Fig. 25. 



REPTILES, serpents, 

 lizards, and tortoises, 

 (figs. 24 and 25) the 

 optic lobes and pineal 

 gland preserve the 

 same relations ; but 

 the hemispheres (fig. 

 24, a) are much in- 

 creased in volume, and 

 the olfactory nerves 

 (fig. 25, c) arise from 

 their anterior parts. 

 The hemispheres ap- 

 pear in the form of 

 rolled laminae, and 

 enclose lateral ventri- 

 cles ; on their floor we 

 observe the corpora 

 striata, through which 

 the ascending fibres of the hemispheres are seen to pass. 



[ 95. BIRDS present a stillfurther development, and exhibit a 

 very uniform arrangement of the cerebral parts. 



Fig. 26 represents the brain of a turkey. ^ : ; %; 



The medulla oblongata, d, is considerably : 

 expanded ; a true pons is absent, but some K 

 transverse medullary fibres represent the ru- ^f^"" 

 diment of this cerebcllar commissure. The 

 cerebellum, c, exhibits the middle lobe, 

 with feeble indications of lateral expansi- 

 ons, c*. It is divided into lamellae by trans- 

 verse fissures; portions of the posterior co- 



Fig. 24 represents the brain of a tortoise, 

 in which a, is the hemispheres ; b, the optic 

 lobes ; c, the cerebellum ; d, the pineal gland ; 



5, 9, 10, 11, the pairs of nerves. 



Fig. 25 shows the base of the same brain: 



6, are the hemispheres ; c, the olfactory 

 nerves; 1, the optic nerves; 2, the auditory 

 nerve; c, the medulla oblongata. 



Fig. 26. The brain 

 of a turkey. 



