GENERAL PHYSIOLOGY OF NERVE-TISSUE. 93 



other, curving backward and upward, enters the spinal cord on its posterior 

 or dorsal surface. The former is termed the anterior or ventral root; the 

 latter, the posterior or dorsal root. Each dorsal root presents near its union 

 with the ventral root a small ovoid grayish enlargement known as a ganglion. 

 Both roots previous to entering the cord subdivide into from four to six 

 fasciculi. 



A microscopic examination of a cross-section of the spinal cord shows 

 that the fibers of the ventral roots can be traced directly into the body of the 

 nerve-cells in the ventral horns of the gray matter. The fibers of the dorsal 

 roots are not so easily traced, for they diverge in several directions shortly 

 after entering the cord. In their course they give off collateral branches 

 which, in common with the main fiber, end in tufts which become associated 

 with nerve-cells in both the ventral and dorsal horns of the gray matter. 

 Cranial Nerves. The nerves in connection with the base of the brain 

 are known as cranial nerves; some of these nerves present a similar ganglionic 

 enlargement, and therefore may be regarded as dorsal nerves, while others 

 may be regarded as ventral nerves. Their relations within the medulla 

 oblongata are similar to those within the spinal cord. 



Efferent and Afferent Nerves. Nerves are channels of communication 

 between the brain and spinal cord, on the one hand, and the skeletal muscles, 

 glands, blood-vessels, visceral muscles, skin, mucous membrane, etc., on 

 the other. Some of the nerve-fibers serve for the transmission of nerve 

 energy from the brain and spinal cord to certain peripheral organs, and so 

 accelerate or retard, augment or inhibit their activities; others serve for the 

 transmission of nerve energy from certain peripheral organs to the brain and 

 spinal cord which gives rise to sensation or other modes of nerve activity. 

 The former are termed efferent or centrifugal, the latter afferent or centripetal 

 nerves. Experimentally it has been determined that the anterior or ventral 

 roots contain all the efferent fibers, the posterior or dorsal roots all the afferent 

 fibers. 



The Peripheral Endings of Nerves. The efferent nerves as they 

 approach their ultimate terminations lose both the neurilemma and myelin 

 sheaths. The axon or axis-cylinder then divides into a number of branches 

 which become directly and intimately associated with tissue-cells. The 

 particular mode of termination varies in different situations. These termina- 

 tions are generally spoken of as end-organs, terminal organs, or end-tufts. 



In the skeletal muscle the nerve-fiber loses both neurilemma and myelin 

 sheath at the point where it comes in contact with the muscle-fiber. After 

 penetrating the sarcolemma, the axon or axis-cylinder divides into a number 

 of smalt branches which appear to be embedded in a relatively large mass of 

 sarcoplasm and nuclei, the whole forming the so-called "motor plate." 

 Each muscle-fiber possesses one such plate or end-organ in mammalia, 

 several in the frog. (Fig. 46.) 



In the visceral muscle the terminal nerve-fibers derived from sympathetic 

 or peripheral neurons are primarily non-medullated. The axons divide and 

 subdivide and form plexuses which surround the muscle-cell bundles. Fine 

 fibers from the plexuses are given off which ultimately come into relation 

 with each individual cell, on the surface of which they terminate in the form 

 of one or more granular masses. 



