SPINAL CORD. 



47o 



they form Gerlach's neme-network, in which small and larget 

 nerve-fibers exist together. These fibers can be traced to the 

 medullated fibers of the posterior nerve-roots, and also to the 

 processes of the ganglion-cells, thus bringing these cells into con- 

 nection with the posterior nerve-roots through the network. 



The fibers of the anterior horn are directly continuous with the 

 axis-cylinder processes of the ganglion-cells. 



The following illustration (Fig. 275) represents the relations 

 between the nerve-cells, the skin, and a muscle. 



Spinal Nerves. There are thirty-one pairs of spinal nerves, 

 which are distributed to the neck, trunk, and extremities. Each 



M 



FIG. 275. Schematic diagram of a sensorimotor reflex arc according to the 

 modern neuron theory ; transverse section of spinal cord ; mN, motor neuron ; sN, 

 sensory neuron ; (7 1 , nerve-cell of the motor neuron ; (7 2 , nerve-cell of the sensory 

 neuron ; d, dendrite; n, neuraxis of both neurons ; , telodendrons ; Jf, muscle-fiber; 

 h, skin with peripheral telodendron of sensory neuron (Bohrn and Davidoff ). 



of these arises by two roots ; an anterior or motor, and a posterior 

 or sensory root. 



Anterior Roots. These are traced through the anterolateral 

 column to the cells of the gray matter of the anterior cornu. 

 Around the cells is an interlacement of ramified nerve-endings, 

 which come especially from the collaterals of the posterior root- 

 fibers and from those of the fibers of the white substance of the 

 cord. 



Posterior Roots. These are characterized by the presence upon 

 each of a ganglion, except the posterior root of the first cervical 

 nerve, which frequently possesses no ganglion. These roots have 

 their origin in the cells of the ganglion, and pass into the postero- 

 lateral column, some entering the marginal bundle of Lissauer, 



