Nervous System and Sense Organs 199 



the cerebral cortex, the diencephalon, and the mesencephalon. The cor- 

 pus striatum is composed of both gray and white matter and is much 

 more conspicuous in the brains of reptiles and amphibians than in 

 mammals. 



The second region of the brain, the diencephalon, consists of the 

 thalamus, hypothalamus, the genicidaie bodies, and the pineal body. 

 The chief function of the thalamus is to carry impulses into the cere- 

 brum. All afferent impulses, except those from the olfactory lobes, 

 must pass through the thalamus before going to the cerebrum. The 

 hypothalamus, which is just below the thalamus, receives nerve fibers 

 from the thalamus, the cerebrum, and other regions and sends other 

 fibers to the mesencephalon and the myelencephalon. In addition, this 

 portion acts as an integrator of the involuntary reactions of the auto- 

 nomic nervous system. In this function, it helps in the maintenance 

 of body temperature, preservation of water balance, and regulation 

 of bodily changes which occur during emotional upsets. Lesions which 

 may occur in this region often result in the individual's having violent 

 uncontrolled fits of anger. 



Just ventral to the hypothalamus is the hypophysis or pituitary 

 body, an endocrine structure whose function will be discussed later. 



Immediately in front of the hypophysis the optic nerves cross as 

 the optic chiasma. Most of these nerve fibers terminate in the genicidate 

 bodies rather than in the mesencephalon. From the geniculate bodies, 

 nerve tracts relay the visual impulses to the cerebral cortex where they 

 are interpreted. The pineal body is a small dorsal projection of the 

 diencephalon. It has no known function, although it corresponds in posi- 

 tion to the third eye of some reptiles and fish. 



The mesencephalon is the least modified of all the divisions of the 

 brain. Essentially it consists of fiber tracts which connect the anterior 

 and posterior parts of the brain and cord. In mammals on the dorsal 

 surface is the corpora qiiadrigemina, a four-lobed body. The anterior 

 pair of these lobes is concerned with visual reflexes, the posterior 

 pair with hearing. 



Three main regions form the metencephalon: the pons, the cere- 

 bellum, and three large nerve tracts. Of these three the cerebellum 

 is the most conspicuous. This oval structure has a central striated 

 portion, the vermis, and two lateral expanded portions, the hemispheres. 

 Like the surface of the cerebrum, that of the cerebellum is composed of 

 gray matter or nerve cell bodies. A section through the cerebellum re- 

 veals the treelike arrangement of the white matter which bears the de- 



