124 THE HORSE IN HEALTH AND DISEASE 



periphery to tlie central nervous system and efferent impulses 

 from this to the muscles and other of its servants. In order to 

 perform its function the nerve-fiber must have direct continuity 

 with its nerve-cell. 



The rate of conduction along a nerve-fiber, although very rapid, 

 may be measured by an especially designed electromagnetic appa- 

 ratus. A nerve of a certain muscle is stimulated at two points a 

 known distance apart, and the time which elapses between the 

 contraction of each is determined. Many measurements have 

 demonstrated that an impulse passes along a nerve at a rate 

 exceeding 100 feet in a second; so the longest paths in the body are 

 traversed almost instantaneously. 



The direction in which a nerve-fiber conducts may be deter- 

 mined by either cutting it or stimulating it and noting the result 

 of the stimulation. Nerves which conduct impulses from, the 

 central nervous system outward are termed "efferent" nerves. 

 They always belong to the ventral nerve-roots of the spinal cord. 

 Nerves conducting in the opposite direction are known as "affer- 

 ent" nerves and belong to the dorsal roots of the gray matter of 

 the spinal cord. 



Efferent nerves may either increase the activity of the parts 

 which they supply or diminish or inhibit activity. The former class 

 are termed "augmentor" nerves; the latter, "inhibitory" nerves. 

 Augmentor nerves are divided into the motor nerves, which act 

 on muscles; secretory Jierves, which act on secretory glands, and 

 vasoconstrictor nerves, which narrow the lumen of the blood-vessels. 

 Inhibitory nerves are divided in a manner similar to the above into 

 musculo-inhibitory , secreto-inhihitory , and vaso-inhihitory nerves. 



Afferent nerves conduct impulses from the outer parts of the 

 body to the central nervous system. They are divided into sen- 

 sory nerves, which when stimulated cause sensations, and excito- 

 refl.ex nerves, which give rise to the so-called reflex actions. . These 

 two are not distinct from each other, for at one time they will 

 cause a sensation and at another time a reflex action without sensa- 

 tion. 



The function of the cerebrum is to govern both consciousness 

 and intelligence. Removal of this portion of the brain does not 

 interfere with eating or sleeping, but the animal cannot associate 

 its sensations. There are certain areas which can be mapped out 

 on the cerebrum of the dog for movements of the eye, the vocal 



