THE NERVOUS SYSTEM 125 



cords, and movements in response to commands. The human 

 brain has been mapped out in great detail, with results which show 

 that the intricacies of it are almost beyond conception. 



The function of the cerebellum is to act as the center for nerves 

 of co-ordination like those in the semicircular canals of the ears, 

 and to the muscles, tendons, and joints that are concerned with 

 movements and postures of the body. When its physiologic 

 activities are interfered with the animal reels when it moves and 

 acts as if intoxicated. A serious injury to the cerebellum may 

 cause the patient to walk or spin around in a circle or turn somer- 

 saults. 



The functions of the pons and medulla are closely related. 

 These parts are remarkable for the great number of reflex centers 

 they contain — centers for the heart movements, respiration, swal- 

 lowing, vomiting, and for closure of the eyelids are located in these 

 parts of the brain. 



The functions of the spinal cord and nerves are varied. With 

 the exception of a part of the face all of the body is endowed with 

 sensory nerve-fibers from the spinal cord. The nerve-fibers of the 

 ascending and descending tracts in both the white and gray matter 

 of the cord conduct sensations to and from the brain, and, in con- 

 junction with the spinal nerves, from one part of the body to other 

 parts. This latter function is dependent upon the reflex centers 

 which are located at different points in the spinal cord. In the 

 lumbar region there are special reflex centers for impulses from the 

 bladder, rectum, and the sexual organs. 



A reflex action is one produced independently of the will. It 

 is always of a distinct purposeful character, and is made possible 

 by a neural arc in the spinal cord. The mechanism involved 

 consists of sensory fibers, to conduct to the nerve-center in the 

 spinal cord, the cross-over or connecting fibers, and the motor 

 fibers to carry the impulse to the muscles. Hough and Sedgwick 

 say that it is a common error 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-vohtional 

 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 



