56 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



of naked-eye vision, and finally terminate mainly in two 

 ways, viz., some in muscular tissues and some in sensitive 

 surfaces and sense organs. 



Structure of Nerves. — A nerve is a bundle of 

 slender fibers of extreme fineness lying parallel and in- 

 vested by a membrane of fibrous tissue — neurolemma. 

 The size of the fibers varies from T -^ to j^-J-no- of an 

 inch, or even less. The coarsest are the motor fibers 

 and the finest the sensory fibers of the optic nerve. The 

 number in a nerve of -^ inch in diameter may be a mil- 

 lion or more. Each fiber consists of a central medullary 

 part and an investing sheath. Each fiber may be con- 

 tinuous from a cell in the central gray matter to its ter- 

 mination in the tissue, but this is probably not true of all. 

 The cerebral cells connect with the surface only through 

 a chain of several cells in the thalamus, the medulla, and 

 the spinal column. A branch of a nerve therefore con- 

 sists of a number of fibers separated and invested as 

 before, but without branching of the fibers themselves, 

 except at the extreme end where they may form dendrites. 

 Thus we may regard each fiber as continuous, one end 

 terminating in a central cell, the other in a tissue. 



Function of Nerves. — Nerve fibers are of two 

 kinds, sensory and motor. The one transmits external 

 impressions inward to the nerve center (afferent), and may 

 or may not awaken consciousness; the other transmits 

 internal impulses outward (efferent), and determines mus- 

 cular contraction. These two kinds may lie side by side 

 in the same nerve undistinguishable from one another 

 except that the motor is usually larger. The termina- 

 tions of the one are centrally in a sensory cell of the 

 central gray matter, and peripherally by a peculiar end- 

 ing in a sensitive surface or a sense organ ; the termina- 

 tions of the others are centrally in a motor cell of the 

 central gray matter, and peripherally in a muscular fiber. 



