NERVOUS SYSTEM 



863 



velop so far as to reduce the third ventricle to a narrow slit, even causing 

 the two edges to unite. This forms the soft commissure or intermediate 

 mass of the mammalian brain. The sense of hearing is not very acute. 



-ol.b. 



'Cr,t. 



x a.ch. p. 

 ' ,> l.v. 



op. t*. 



A. B. 



Fig. 484. 



A. The brain of a rabbit, seen from above with part of the right cerebral hemisphere cut 

 away, a.c.q., Anterior corpus quadrigeminum ; a.ch.p., anterior choroid plexus ; cb., cerebellum ; 

 cer.h., cerebral hemisphere ; crt., cortex ; ft., flocculus ; fr.l., frontal lobe of cerebral hemisphere ; 

 l.v., lateral ventricle ; lat.l., lateral lobe of cerebellum ; m.o ., medulla oblongata ; occ.L, occipital 

 lobe of cerebral hemisphere ; ol b., olfactory bulb ; op.th., optic thalamus ; p. b., pineal body ; 

 p.c.q., posterior corpus o.uadrigeminum ; par.L, parietal lobe of cerebral hemisphere ; r.3, roof of 

 third ventricle ; sp.c., spinal cord ; Sy.f., Sylvian fissure ; tp.l., temporal lobe of cerebral hemi- 

 sphere ; ver., vermis. 



. B. The brain of a rabbit from below, c.al., corpus albicans ; fl., flocculus ; fr.l., frontal 

 lobe of the cerebral hemisphere ; hip.L, hippocampal lobe ; m.ob., medulla oblongata ; ol.b., olfactory 

 bulb; ol.t., olfactory tract; p.V., pons Varolii ; pit., pituitary body; rh f.. rhinal fissure; 

 Sy.f., Sylvian fissure ; tp.l.* temporal lobe of the cerebral hemisphere ; II.-XII.^ roots of the 

 cranial nerves. (From Borradaile. ) 



The tympanic membrane is thin and exposed, and is connected with the 

 auditory organ by a slender columellar bone. The sense of smell is the 

 keenest of the senses in the turtle, both in the air and in the water. In 

 correlation with the keen olfactory sense, the olfactory lobes of the brain 

 arc highly developed. In the crocodile (Fig. 483), the brain is decidedly 

 advanced in structure for a reptilian brain. The large cerebral hemi- 

 spheres are especially noteworthy. The tympanic membrane is sunk in 

 a pit. This is a tendency that is carried much further in the birds and 

 mammals. 



AVES 



The brain is very short and broad; the cerebrum is large but not 

 convoluted. The cerebellum is very large and complex. All three 

 flexures are partially retained throughout life. The optic lobes are well 

 developed. The olfactory lobes are rudimentary, indicating a poor sense 

 of smell. The olfactory epithelium is poorly developed, and sense of 



