304 



THE INTERNAL ENVIRONMF.NT OF THE BODY 



Part III 



gray ma'lVar 

 of cerebral 

 corVe 



a qijrus 

 a sulcus 



h.xjpo^hQlainus 



optic chiasma 



pi'luitarij qland-fpars 



[pars p 



ost. 



phorioid 

 us 



CGn'tral 

 carxal of 

 spinal cord. 



Fig. 16.21. The right half of the human brain. A piece has been cut from the 

 front of the cerebral hemisphere in order to expose the lateral ventricle. This 

 ventricle and its mate in the left hemisphere, and the central third and fourth 

 ventricles have developed from the central canal of the primitive brain and cord. 

 The hypothalamus that forms part of the floor of the third ventricle is believed to 

 have an important part in controlling the secretion of the adrenocorticotrophic 

 hormone (ACTH) under conditions of stress. The cut in the cerebral hemisphere 

 reveals the thickness of the cortex, the shaded gray matter whose area is greatly 

 increased by the folds that are absent in lower animals. It is estimated that there 

 are 10,000 million nerve cells in the cortex of the human brain, each one having 

 synaptic connections with several others. The number of pathways in these highest 

 centers of the brain is beyond imagination. (Courtesy, Ham: Histology, ed. 2. 

 Philadelphia, J. B. Lippincott and Co., 1953.) 



4. Hindbrain {cerebellum). On its dorsal side the hindbrain is com- 

 posed of the cerebellum and on its ventral side, of the floor of the ancient 

 brain stem. The pons, a bridge of nerve fibers including those that connect 

 the cerebellum and cerebral hemispheres, forms a part of the floor of the 

 fourth ventricle. Some of the main functions of the cerebellum are the main- 

 tenance of unconscious muscular coordination, and the preservation of 

 muscular tension or tonus. It contains numerous connections with the eyes, 

 ears, muscles, joints, and other parts of the body. 



5. Hindbrain {medulla). The white appearance of the hindbrain is due 

 to the fibers of the nerve cells being on the outside as they are in the spinal 

 cord. It is the great passageway for nerve fibers that extend along the sides 

 and form the swollen cords of the pyramids. Its cavity is the fourth ventricle 

 and its thin roof, the choroid plexus, is one of the main sources of cerebro- 

 spinal fluid. 



