422 



ORGAN SYSTEMS OF MAN 



the myelencephalon. There is a clear rela- 

 tionship between the size of a particular 

 part of the brain of an animal and its im- 

 portance in the life of tlie animal. In the 

 lower vertebrates, such as fish and frogs, 

 the olfactory lobes are large, because this is 

 probably the most important sense these 

 animals possess, whereas in birds where the 

 sense of smell is only poorly developed this 

 part of the brain is proportionately small 

 (Fig. 16-20). In the lowest vertebrates the 

 cerebrum is non-existent, or almost so, 

 whereas in the birds it begins to dominate 

 the anterior end of the brain and in mam- 

 mals it overgrows all other parts to become 

 the most prominent part. In man, this trend 

 is carried to the most extreme point of de- 

 velopment. 



Emphasis of various parts of the brain 

 seems to be associated with the kind of life 

 its owner leads. For example, birds and fish, 

 which move in three dimensions, require 

 especially good muscular coordination to 

 balance their bodies, and since this nervous 

 center is located in the cerebellum, this 

 portion of the brain is well developed ( Fig. 

 16-20). Reptiles and land dwelling mam- 

 mals (mouse and man), on the other hand, 

 have a rather poor sense of balance because 

 they move in two dimensions for the most 

 part and do not require the coordination 

 that fish and birds do for balancing their 

 bodies in a fluid or gaseous environment. 



The cord takes care of the simplest activ- 

 ities in the organism and its anterior end, 

 the medulla, still retains that function to a 

 high degree even in man. It is here that the 

 reflexes for such basic activities as respira- 

 tion and heart action center. The cerebel- 

 lum functions in muscular coordination but 

 the cerebrum, the last region to evolve, is 

 the center of such highly complex activi- 

 ties as thought and reasoning. 



As the vertebrate brain has evolved there 

 was a gradual shift of function from the 

 lower part, the brain stem, to the higher 

 part, the cortex. This shift can be demon- 

 strated experimentally. When the cerebrum 



of a frog is removed its normal activities 

 are influenced only slightly. It jumps nor- 

 mally when stimulated and it can swim in 

 a perfectly normal fashion (Fig. 16-21). 

 Even a "decerebrated" reptile shows very 

 little concern about its loss. Such an opera- 

 tion on a bird or mammal, however, brings 

 about striking changes. The ability to loco- 

 mote is destroyed and all actions which re- 

 quire considerable muscular coordination 

 are lost. This simply means that the higher 

 vertebrates have shifted their nerve centers 

 from the lower brain stem to the cerebral 

 cortex. 



This shift has given these higher forms 

 much greater plasticity in the control of 

 their muscle coordination. For example, 

 when certain muscles in man have lost their 

 nervous connection with the brain in the 

 disease known as facial paralysis, functional 

 cranial nerves passing to relatively weak 

 and less useful muscles can be transplanted 

 to the larger more important muscles and 

 eventually become functional through long 

 retraining. In other words, impulses can be 

 sent to a muscle via a wholly new nerve 

 with the result that the muscle can eventu- 

 ally respond in its usual manner. In man, 

 the cortex so completely dominates the 

 body that it is physiologically possible by 

 intense effort to learn to move muscles 

 in almost any manner. This is well illus- 

 trated by the many human feats performed, 

 activities that could never be executed by 

 other animals because their nervous sys- 

 tems are constructed in such a way as to 

 make it impossible. This shift of control 

 to the cortex accounts for man's great versa- 

 tility and is one of the major reasons for his 

 success. It has also made possible the devel- 

 opment of our type of society. 



As the cerebrum increased in size in rep- 

 tiles, birds, and mammals, it became neces- 

 sary to increase the surface area whfle 

 retaining a reasonable volume. This has 

 been done by the formation of wrinkles 

 or convolutions. Starting in the lower mam- 

 mals with only very few convolutions, the 



