134 



HISTOLOGY 



the segments of the body. Thus they form the spinal ganglia. A typical 

 cell of a spinal ganglion is at first round, but later becomes bipolar by send- 

 ing out two processes, one toward the periphery and the other toward the 

 medullary tube. These processes grow out from opposite ends of the cell 

 (Fig. 126). With further growth the nucleated cell body passes to one 

 side of the prolongations, with which it remains connected by a slender 

 stalk. Such T-shaped cells are characteristic of the spinal ganglia. The 

 fibers which grow toward the medullary tube enter its outer part and then 

 bifurcate, sending one branch toward the brain and the other down the 

 cord. These longitudinal fibers form distinct oval bundles just within 



Bipolar cells. T-cell. the C0rd > OnG On cither side ( Fi g I2 5> E ) 



Since these bundles receive accessions of 

 fibers from every spinal ganglion, they en- 

 large as they approach the brain. The 

 fibers of the oval bundle branch freely at 

 their terminations, and along their course 

 they give off collateral branches, which enter 



FIG. 126. SPINAL GANGLION CELLS. . 



The bipolar forms are from a chick the deep substance of the cord. The periph- 



embryo incubated six days. i .ei_ r j.i i T 



eral fibers from the spinal ganglia grow out- 

 ward through the mesenchyma, and terminate in sense organs or sen- 

 sory endings, which will be described presently. The fibers of the spinal 

 ganglia are essentially sensory or afferent, conveying impulses from the 

 periphery toward the cord, and up the cord toward the higher nervous 

 centers. 



The efferent or motor fibers arise chiefly from cells, the bodies of which 

 remain within the central nervous system. Each of these nerve-forming 

 cells, or neuroblasts, sends out one long process called a neuraxon (or axone) . 

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

 surface, in bundles which unite to form the ventral roots. The ventral 

 roots correspond in number with the dorsal roots, which are the bundles 

 of sensory fibers passing into the cord from each spinal ganglion. Periph- 

 erally the ventral root joins the bundle of fibers growing outward from 

 the spinal ganglion, and the two together form a spinal nerve. Every 

 spinal nerve consequently 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 fundamental facts which have just been reviewed eluded anatomists for 

 centuries. The nerves, extending from the brain and cord to all the important organs, 

 were regarded as tubes, conveying a vital fluid necessary for organic activity; when 

 this supply was cut off, the organs ceased to perform their functions. Thus if nerves 

 to the skin were destroyed, the skin became insensible; or if those to muscles were 

 cut, the muscles could not contract. The possible existence of sensory and motor 

 nerves with different functions was debated and generally rejected, until Charles Bell 

 proved conclusively that "nerves entirely different in function extend through the 



