120 THE INDIVIDUAL ORGANISM 



generally recognized. The first, or sympathetic division, comprises those 

 autonomic fibers that arise in the thoracic and lumbar regions of the 

 spinal cord. The second, or parasympathetic division, includes those fibers 

 that come from the brain and those arising in the sacral region of the 

 cord. In general, the actions of the sympathetic and parasympathetic 

 nerves are different, and commonly antagonistic, and the visceral organs 

 for the most part receive a double (sympathetic and parasympathetic) 

 autonomic innervation. Thus in the instance of the heart, the vagus nerves 

 (parasympathetic) slow or inhibit its action, while the cervical sym- 

 pathetic nerves accelerate the heartbeat. Glands, smooth muscles of the 

 viscera, and smooth muscles in the blood vessels are under similar dual 

 nervous control. 



Second-level response — the lower brain centers. We have seen 

 that the simplest known activity of the nervous system is the conduc- 

 tion of the nerve impulse over a simple reflex arc. If we turn once more 

 to our experimental animal, the frog, we can demonstrate other and 

 more complex actions. If another specimen is prepared in which the 

 brain stem and cerebellum are left intact but in which the cerebrum is 

 removed, more, but not all, of the normal responses of the animal to 

 external stimuli are possible. 



A frog thus deprived of its cerebrum can jump and swim; it can breathe 

 and swallow. If food is placed in the mouth, it not only will be swallowed 

 but also will be digested. However, no attempt is made to capture food, 

 even if the animal is starving; food is not recognized as such. In general, 

 "decerebrated" animals make no response at all unless the stimulus can 

 be carried directly to the spinal cord. 



Such experiments as these are, of course, out of the question with man; 

 but the accumulated observations from accidents, operations, and dis- 

 eased nervous systems and from a considerable variety of noninjurious 

 and nonpainful experiments on volunteer subjects show the same results 

 in man. 



We have seen that the brain is composed of the large cerebral hemi- 

 spheres and the less conspicuous cerebellum and brain stem — this latter 

 portion being divided into the medulla, pons, midbrain, and thalamus. 

 We can get a better idea of the relationships of these parts by examining 

 the median surface of a brain that has been divided longitudinally into 

 right and left halves. Such a section is shown in Fig. 7.10. Here it was 

 necessary to pull the cerebral hemispheres gently apart and then to 

 divide the rest of the brain longitudinally with a sharp knife. 



The nervous mechanisms of the second level directly involve the spinal 

 cord and all the portions of the brain shown in Fig. 7.10 except the cere- 

 brum. The medulla, pons, midbrain, and thalamus act, like the spinal 

 cord, partly in the capacity of conductors. The white matter of these 



