GENERAL PHYSIOLOGY OF NERVE-TISSUE 95 



cerebrosides or galactosides (nitrogen-holding bodies, free from phos- 

 phorous, compounds of a glucoside character, as shown by their yielding on 

 hydrolysis the reducing carbohydrate galactose), (c) phosphatids (com- 

 pounds containing both nitrogen and phosphorus, e.g., lecithin, kephalin, 

 sphingo-myelin), inorganic salts, and a series of nitrogen-holding bodies 

 such as creatin, xanthin, urea, leucin, etc. As to the metabolism that is 

 taking place in nerve-cells and fibers, practically nothing definite is known. 

 That such changes, however, are taking place would be indicated first by the 

 blood-supply, and second by the fact that withdrawal of the blood-supply is 

 followed by a loss of irritability. The metabolism of the central organs of 

 the nerve system is more active and extensive. In this situation any with- 

 drawal of blood from compression or occlusion of blood-vessels is followed 

 by impairment of nutrition and loss of function. 



THE RELATION OF THE PERIPHERAL ORGANS OF THE NERVE 

 SYSTEM TO THE CENTRAL ORGANS 



Spinal Nerves. The nerves in connection with the spinal cord are 

 thirty-one in number on each side. If traced toward the spinal column, it 

 will be found that the nerve-trunk passes through an intervertebral foramen. 

 Near the outer limits of the foramina each nerve-trunk divides into two 

 branches, generally termed roots, one of which, curving slightly forward and 

 upward, enters the spinal cord on its anterior or ventral surface, while the 

 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 



