538 TEXT-BOOK OF PHYSIOLOGY 



nerves, so the nerve-cells of which the nerves are integral parts may be 

 classified physiologically as motor, vaso-motor, secretor, viscero-motor and 

 pilo-motor. Collections or groups of such cells are conventionally termed 

 " centers." 



The Relation of the Dorsal Roots of the Spinal Nerves to the Intra- 

 spinal Cells.- Histologic investigation has shown that the cells of the 

 ganglia on the dorsal roots of the spinal nerves give origin to an axonic 

 process which soon divides into two branches, one of which passes in a 

 peripheral, the other in a central direction. 



The peripherally coursing branches are distributed in part to skin and 

 mucous membrane, and in part to muscles, tendons, and joints. The 

 nerves distributed to the skin receive their stimulation from external 

 agents and are therefore termed exteroceptive nerves; the nerves dis- 

 tributed to muscles, tendons and joints, receive their stimulation from 

 changes taking place in and around these organs and are therefore termed 

 proprioceptive nerves. (See Figs. 224 and 229.) 



The centrally coursing branches pass to and enter the cord. The 

 exteroceptive nerve-fibers enter the spinal cord both by way of the dorsal 

 funiculus and by way of the dorsal horn where some become directly asso- 

 ciated with the correlated neuron and thus indirectly with cells in the 

 ventral horn, while others become associated with afferent or receptive 

 cells in the base of the dorsal horn. The proprioceptive fibers after enter- 

 ing the cord pass through the fibers composing the dorsal funiculus before 

 entering the gray matter. Some of the fibers become associated either 

 directly or indirectly with cells in the ventral horns, others with the cells 

 of Clark's vesicular column, while still others ascend the dorsal funiculus 

 of the same side to the lower level of the medulla oblongata where they 

 terminate around the nerve-cells composing the clavate and cuneate nuclei. 



THE FUNCTIONS OF THE GRAY MATTER 



Anatomic investigation has demonstrated that the segments of the 

 spinal cord are associated through their related spinal nerves with the 

 organs and tissues of definite areas of the body. Physiologic investiga- 

 tion has also demonstrated that the groups of cells in the ventral horns, 

 viz.: the efferent or the emissive cells are the immediate sources of the 

 nerve energy that excites activity in skeletal muscles, glands, vascular, 

 and to some extent visceral muscles. 



The discharge of their energy may be caused : 



1. By variations in the composition of the blood or lymph by which they 



are surrounded or as the outcome of a reaction between the chemic 

 constituents of the lymph on the one hand and the chemic constituents 

 of the nerve-cell on the other hand. The excitation of the cell thus 

 occasioned is termed automatic or autochthonic excitation. 



2. By the arrival of nerve impulses, coming through afferent nerves from 



the general periphery, skin, mucous membrane, etc. 



3. By the arrival of nerve impulses descending the spinal cord from cells in 



the cortex of the cerebrum or subordinate regions. 



The excitation in the former instance is said to be reflex or peripheral 

 in origin; in the latter instance direct or central in origin. In the direct 

 central or cerebral excitations the skeletal muscle movements are due to 



