MAN 191 



bral hemispheres in the anthropoid apes and man that 

 they cover and ahnost envelop the brain stem and con- 

 stitute the preponderant mass of the brain as a whole. 



But in all this multiplying of nerve cells and com- 

 pounding of the number of their potential interconnec- 

 tions in the evolution of the brain stem and cerebral cor- 

 tex, no new fimctional features have been added, so far 

 as can be determined, to the basic patents that go back 

 to the very roots of the evolution of the nervous system. 

 As the nervous system has increased in complexity, be- 

 havior has evolved from the simple to the complex sim- 

 ply by grafting the new on to the old, so that the central 

 nervous system of the mammals— the cerebral cortex, 

 brain stem and spinal cord— is in essence a hierarchical 

 system which works the way it does by virtue of the 

 fact that the new can dominate the old. 



Many of the activities of the body— circulation, diges- 

 tion, secretion, excretion, body-temperature control— are 

 taken care of by the autonomic or Vegetative' nervous 

 system, and normally these activities have no counter- 

 part in consciousness, so that a man remains wholly un- 

 aware of them. But neither is a man aware of the in- 

 finitude of automatic operations involved in deliberate 

 or voluntary action. In such a seemingly simple act as 

 walking, hundreds of thousands of neural pathways 

 make their contribution to each step: sensory impulses 

 from the soles of the feet, the joints and tendons of the 

 ankles, legs, thighs, back, arms, and neck, and still others 

 from the semicircular canals of the middle ear as well 

 as visual impulses from the eyes, converge on the brain 

 stem and cerebellum to eflFect the perfect co-ordination 

 of an equally large number of motor impulses activating 

 millions of muscle fibers. Locomotion is in fact a highly 

 complicated performance that is achieved smoothly and 

 easily only because the organism does not have to think 

 about the multitudinous details. As pointedly phrased by 

 Mrs. Edward Craster: 



