16^ 



PRINCIPLES OF ANIMAL BIOLOGY 



secLitively, but all related to one another. The reflex mechanism is 

 sufficiently complicated to care for two or more complex series of reflex 

 actions at the same time. For instance one may swim and talk, or play 

 the piano and sing. In some of these instances part of the action may be 

 voluntary, the remainder involuntary. 



Functions of the Spinal Cord. — The spinal cord acts as a system of re- 

 flex centers which control the activities of various glands, visceral organs, 

 and skeletal muscles. In addition to this function it serves as a pathway 

 to and from the brain. In man more than half a million neurons enter 

 the cord through the dorsal roots of the spinal nerves. These bring in 

 afferent impulses from the peripheral endings of sensory cells. By means 



of neurons which lie in the white 

 portion of the cord and which end in 

 the brain, impulses may be carried 

 upward to the brain while a multi- 

 tude of other neurons which origin- 

 ate in the brain form the pathway 

 of impulses from the brain to the 

 ventral roots of the spinal nerves. 

 In terms of a telephone system the 

 spinal cord may be likened to a series 

 of centrals for the handling of local 

 messages and to a great trunk line 

 which transmits long distance 

 messages arising at any point on the 

 periphery of the system or at the 

 great central station, the brain. 

 Unlike the telephone system, how- 

 ever, messages (impulses) can pass 

 only in a certain direction on the 

 line (neurons). Hence in every re- 

 flex arc there is a neuron for carrying 

 in the impulse initiated by the stimulus and another neuron for carrying 

 out the impulse from the controlling center to the organ which executes 

 the appropriate action. 



Functions of the Cerebrum. — The cortex of the cerebrum, that is, the 

 layer of gray substance which covers the organ and dips down into the 

 furrows, is the seat of intelligence and of conscious sensations. It is 

 the organ of memory associations. In higher animals it controls voluntary 

 motions. In the cerebrum the neurons of one side (right, for instance) 

 give rise to axons which cross to the other side (left) as they extend down to 

 lower centers, and there terminate around cells whose axons in turn go to 

 the muscles (left). Hence the movements of the muscles of one side of 

 the body arc controlled by neurons located in the opposite side of the 



Fig. 121. — Cells of the cortex of the 

 cerebrum of a cat as shown in a Golgi 

 preparation. 



