74 HUMAN PHYSIOLOGY. 



is surrounded by a sheath of its own, consisting of a single layer 

 of endothelial cells. This delicate transparent membrane, the sheath 

 of Henle, is separated from the nerve-fiber by a considerable space, 

 in which is contained lymph destined for the nutrition of the fiber. 

 Near their ultimate terminations the nerve-fibers themselves undergo 

 division, so that a single fiber may give origin to a number of 

 branches, each of which contains a portion of the parent axis- 

 cylinder and myelin. 



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, mucous membrane, viscera, etc., on the other. Some of the 

 nerve-fibers serve for the transmission of nerve energy or nerve im- 

 pulses 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 char- 

 acteristic 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 muscular 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 in- 

 crease in the rhythmic action of certain organs. 



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

 character of the sensations or other modes of nerve activity to 

 which they give rise," into several groups, as follows : 



