66 HUMAN PHYSIOLOGY. 



CLASSIFICATION OF NERVES. 



Nerves are channels of communication between the brain and spinal cord, 

 on the one hand, and the muscles, glands, blood-vessels, skin, mucus mem- 

 brane, viscera, etc., on the other. Some of the nerve-fibers serve for the 

 t ransmission of nerve energy or nerve impulses from the brain and spinal cord 

 to certain peripheral organs, and so increase or retard their activities; others 

 serve for the transmission of nerve energy from certain peripheral organs to 

 the brain and spinal cord, which gives rise to sensations or other modes of 

 nerve activity. The former are termed efferent or centrifugal nerves; the 

 latter are termed afferent or centripetal nerves. 



The efferent nerves may be classified, in accordance with the character- 

 istic form of activity to which they give rise, into several groups, as follows: 



1. Muscle or motor nerves, those which convey nerve energy or nerve impulses 

 to muscles and give rise to muscle contraction. 



2. Gland or secretory nerves, those which convey nerve impulses to glands, 

 and cause the formation of the secretion peculiar to the gland. 



3. Vascular or vaso-motor nerves, those which convey nerve impulses to blood- 

 vessels, and cause, either by stimulation or inhibition of the mechanism of 

 their walls, a contraction (vaso-constrictors) or dilatation (vaso-dilatators) 

 of the vessel. 



4. Inhibitor nerves, those conveying nerve impulses that cause a slowing or 

 complete cessation of the rhythmic action of organs. 



5. Accelerator nerves, those conveying impulses that cause an increase in the 

 rhythmic action of certain organs. 



The afferent nerves may also be classified, in accordance with the charac- 

 ter of the sensations or other modes of nerve activity to which they give rise, 

 into several groups, as follows: 



1. Sensorifacient nerves, those conveying nerve impulses that give rise in 

 the brain to conscious sensations. They may be subdivided into 



(a) Nerves of special sense e. g., olfactory, optic, auditory, gustatory, 

 tactile, thermal, sensory, muscle those which give rise to olfactory, optic, 

 auditory, gustatory, tactile, thermic, painful, and muscle sensations. 



(6) Nerves of general sense e. g., the visceral afferent nerves those 

 which give rise normally to vague and scarcely perceptible sensations, such 

 as the general sensations of well-being or discomfort, thirst, fatigue, sex, 

 want of air, etc. 



2. Reflex nerves, those which convey nerve impulses to the nerve centers and 

 cause a discharge and transmission of nerve impulses outward through 

 efferent nerves to muscles, glands, or blood-vessels, and thus influence their 



