NERVE TISSUE. 



93 



sions of libers from every spinal ganglion, they enlarge as they approach 

 the brain. The fibers of the oval bundle branch freely at their termination 

 and also give off collateral branches along their course, which enter the 

 deep substance of the cord. The peripheral fibers from the spinal ganglia 

 elongate through the mesenchyma, and terminate in branches applied to 

 cells in the skin or muscle spindles, in ways to be described presently. 

 The fibers of the spinal ganglia are essentially afferent or sensory, and they 

 proceed from sensory cells. 



THE VENTRAL ROOTS. The efferent, motor fibers arise chiefly from 

 cells, the bodies of which remain within the central nervous system. Each 

 of these cells sends out one long process called a neuraxon (axone) . The 

 neuraxons of the motor cells leave the spinal cord, near its ventral surface, 

 in bundles which are 

 segmentally arranged 

 so that they corre- 

 spond with the spinal 

 ganglia. A bundle of 

 motor fibers joins a 

 bundle of peripheral 

 fibers from a spinal 

 ganglion to form a 

 spinal nerve. Every 

 spinal nerve conse- 

 quently has a dorsal 

 (sensory) root, and a 

 ventral (motor) root. 

 The fibers from the 

 two roots travel in the 

 same connective tissue sheath, but otherwise they remain entirely distinct. 

 The motor fibers terminate in contact with muscle cells. Soon after a 

 spinal nerve is formed by the junction of its roots, it divides into a dorsal 

 ramus and a ventral ramus (Fig. 109, E). These rami are mixed nerves 

 (containing both sensory and motor fibers) and supply the skin and muscles 

 of the back and of the lateral body wall respectively. 



Within the cord the motor cells send out a large number of com- 

 paratively short branching processes called dendrites. By means of the 

 dendrites the motor cell is put in communication with the collateral fibers 

 of the sensory cells, and with fibers of commissural cells coming either 

 from other parts of the cord or from the brain. This arrangement, is 

 shown in the diagram Fig. 1 1 1 . A painful stimulus transmitted along the 

 sensory fiber, b, passes through the spinal ganglion into the cord. Through 



Diagram of the spinal cord, showing a motor fiber, a; a sensory fiber, 

 b and c ; and a commissural fiber, d,[f rom the^brain ; coll., collateral 

 fiber; sp. g., spinal ganglion. 



