170 PRINCIPLES OF VETERINARY SCIENCE 



two units in the spinal cord. The winking of the eyelid in re- 

 sponse to excitation of its sensitive retina or cornea well illus- 

 trates a reflex action. The sense mechanism involved consists 

 of sensory nerve endings and fibers, to receive and conduct to 

 the spinal cord ; the cross-over or synapse between the dendrites 

 of the sensory and motor nerve cells, to transmit; motor fibers to 

 carry the impulse to the muscles (Fig. 55) . 



The synapse is composed of the fine fibrils of the central 

 branch of afferent nerve cells. The central branch divides on 

 entering the spinal cord into an ascending and a descending 

 branch, each of which runs for a longer or a shorter distance in the 

 white matter of the cord. These branches give off at right angles 

 to themselves collaterals, each of which enters the gray matter 

 and ends there by breaking up into fine fibrils to form the synapse. 



It is a common error, according to Hough and Sedgwick, to 

 suppose that all actions which are not called forth by the will are 

 reflex. The essential feature of a true reflex is the more or less 

 direct action of the afferent impulses on efferent nerve-cells, and 

 not merely its non-volitional character. There are, in fact, 

 involuntary actions in which the efferent nerve-cells are directly 

 stimulated not by afferent nerve-cells, but by the condition of 

 the blood or in other ways. Such actions are not reflex, though 

 they may be either involuntary or unconscious, or both. They 

 are known, in general, as automatic actions. 



The brain, according to Crile, responds to but one stimulus at 

 a time, although it acts with such rapidity and has such capacity 

 for quick changes that it appears to be capable of inducing a 

 number of acts simultaneously. If all the ceptors of the body 

 could be stimulated at one time, the brain would send out first 

 an impulse to the muscles concerned with the action that has 

 proven most important to the survival of the species. In other 

 words, the stimulus which secures possession first of the trans- 

 mission path is always the stimulus which in the history of the 

 species (phylogenetically) is the most important. For example, 

 stimuli threatening life would take precedence over those presag- 

 ing slight physical discomfort. Thus a horse would respond to a 

 heavy blow before he would to a bite of a fly if these two stimuli 

 were received simultaneously. 



The function of the cerebrum is to govern both consciousness 

 and intelligence. Actions are originated and directed by it. 



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