ATLAS OF NERVE CELLS 



Plate VI. shows a group of neurons in the anterior horn of the spinal cord, the intricate 

 interlacing of both dendrites and neuraxons being clearly seen. 



It has already been stated that the function of these neurons is motor, each cell group 



controlling a single muscle. Muscles act in response 

 to two forms of nervous stimulation, viz., reflex and 

 voluntary, and the mechanism, of these different 

 forms of movement requires separate consideration. 

 Reflex motion is easily understood by the aid 

 of the diagram, Fig. 3. In this figure three levels 

 of the spinal cord are shown, A, B, C, each con- 

 taining two motor cells m, n, o, in the anterior horns, 

 and from these motor cells the motor nerve roots 

 issue. Sensory nerve roots I., II., III., IV., are 

 seen to enter the posterior columns and to bifur- 

 cate, turning both up and down. The downward 

 branch of I. ends in B; the downward branches of 

 II. and III. end in C. All send their upward 

 branches to higher levels than those shown. All 

 give off collaterals at all levels, which pass into the 

 gray matter. Some of these collaterals pass forward 

 to the cell of the anterior horn of the same side 

 and terminate in brush-like expansions about the 

 motor cell. A few collaterals pass to the median 

 gray matter and terminate in brush-like expansions 

 about the commissural cell (c}, whose neuraxon 

 passes to and terminates about the motor cell of 

 the opposite side. A sensory impulse entering the 

 cord at the level B, through the posterior nerve 

 roots II. or III., is sent (i) to the motor cell, , on 

 the same side, and (2) if sufficiently intense to 

 excite the intermediate cell c, to the motor cell of 



A, B, and C, three levels of the cord. 7, 77, 777, and 



IV, posterior nerve roots entering the cord, bifurcating, ascend- the Opposite side also. The Sensory impulse also 

 ing and descending, and sending their collaterals and terminations , , , , , /-* i . 



passes down the cord to level C, where it may 



into the gray matter. These terminals end in brushes around the 



motor cells m, , o. Some terminals end in brushes around the excite (3) motor Cells, O and O, the latter (4) through 



commissural cells, <-, which in turn communicate impulses to 



the motor cells of the opposite side, m, n, o. the medium of commissural cell c. It also passes 



FIG. 3. 

 FIG. 3. Diagram to illustrate the reflex action of the spinal cord. 



