168 FEINCIPLES OP VETEEINAET SCIENCE 



In order to perform its function the nerve-fiber must have direct 

 continuity with its nerve-cell. 



The rate of conduction of the impulse along a nerve-fiber, 

 although very rapid, may be measured by an especially designed 

 electromagnetic apparatus. A nerve of a certain muscle is stimu- 

 lated 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. A nerve can conduct impulses in only one 

 direction, viz., either to the center or to the periphery. Nerves 

 which conduct impulses from the central nervous system out- 

 ward 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 afferent nerves and belong 

 to the dorsal roots of the gray matter of the spinal cord. 



Efferent nerves transmit impulses from the nerve centers to 

 the muscles, glands, and blood-vessels. They may either in- 

 crease the activity of the parts which they supply or diminish 

 activity. Nerves which perform the former kind of work are 

 termed augmentor nerves; the latter, inhibitory nerves. 



Augmentor nerves are divided into the motor nerves, which act 

 on muscles; secretory nerves, which act on secretory glands; 

 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-inhibitory, and vaso-in- 

 hibitory 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- 

 reflex 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 with- 

 out sensation. 



A sensation is the consciousness that an afferent nerve has 

 been stimulated. Sensations are of various forms; important 

 ones are the sensations of heat, cold, hunger, thirst, taste, smell, 

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