450 STUDIES IN ADVANCED PHYSIOLOGY. 



1. Arrangement of the Motor Neurons. The arrange- 

 ment of the motor neurons is about as follows: In the 

 cortex of the brain near the fissure of Rolando are certain 

 large cells which give off small branches in various direc- 

 tions and usually one long branch, the nerve fiber proper. 

 This extends from the cortical region through the crus 

 cerebri of its side, and in the medulla crosses to the opposite 

 side of the cord. It then descends through the spinal cord 

 in the so-called lateral column of that side, commonly des- 

 ignated the crossed pyramidal tract, and finally enters the 

 anterior horn of the spinal cord and ends there in the fine 

 network of dendrons, which closely invest similar short 

 dendrons of the motor cells of the anterior horn. These 

 motor cells are the cell bodies of the second neurons, and 

 their nerve fibers extend from the anterior horn through the 

 anterior roots of the spinal nerve to the muscles in ques- 

 tion. In other words, from the point in the brain where the 

 volition arises to the point in the muscle where the con- 

 traction is produced there are two neurons, one reaching 

 from the brain, where the cell body is, through the cord to 

 the anterior horn, the second neuron reaching from this 

 point to the muscle. This is the usual course of the motor 

 neurons. 



A second course is, however, possible. Many motor 

 fibers arising in the cortex do not cross to the opposite side 

 in the medulla, but descend the spinal cord on the same 

 side along the anterior tracts, and then along in the course 

 of the spinal cord cross to the other side, through the an- 

 terior commissure of the cord. It will be seen, however, 

 that all these motor fibers finally reach the opposite side of 

 the cord, and the difference between the fibers in the 

 lateral column and those in the direct pyramidal tract is a 

 secondary one. The fact that some cross in the medulla 

 and others in the cord further down is no real difference. 

 In either case these neurons from the brain end in the an- 

 terior gray horns, and there connect with the second motor 

 neurons which reach to the muscles. The general arrange- 



