54 8 TEXT-BOOK OF PHYSIOLOGY 



The reason for the development of this pathway is apparent from the 

 following considerations: 



At birth the child is capable of performing all the functions of organic 

 life, such as sucking, swallowing, breathing, etc. It is, however, deficient 

 in psychic activity and in volitional control of its muscles. Its movements 

 are therefore largely, if not entirely, reflex in character. 



Embryologic and histologic examination of the spinal cord and medulla 

 show that so far as their mechanisms for independent physiologic activities 

 are concerned both are fully developed. Similar investigations of the cere- 

 bral hemispheres and of the nerve-fibers which bring their nerve-cells intc 

 relation with the spinal segments show that the cells of the cortex are not onl) 

 immature, but that their descending axons are incompletely invested witli 

 myelin. With the growth of the child, psychic life unfolds and volitional 

 control of muscles is acquired. Coincidently the cells of the cerebral cortes 

 grow and develop and the fibers become covered with myelin. 



The nerve-fibers, which have their origin in the cells of the cerebral cortex, 

 and which terminate in tufts around the cells in the anterior horns of the 

 gray matter of the spinal segments, are to be regarded as long commissural 

 tracts uniting and associating these two portions of the central nerve system, 



Experimental investigations and observations of pathologic lesions 

 accord with the view that physiologically these fibers are efferent pathways 

 for the transmission of motor or volitional impulses from the cortex to the 

 spinal segments. The nerve-cells in which the motor impulses originate 

 are located for the most part, as will be fully stated later, in the central 

 portion of the cortex of the cerebral hemispheres in the neighborhood of the 

 central or Rolandic fissure. The axons of these cells from each hemisphere 

 descend through the corona radiata to and through the internal capsule, 

 along the inferior surface of the crura cerebri, behind the pons to the medulla, 

 of which they constitute the anterior pyramids (Fig. 234). At this poinl 

 the pyramidal tract 1 of each side divides into two portions, viz.: 



1. A large portion, containing from 80 to 90 per cent, of the fibers, which 



decussates at the lower border of the medulla and passes downward in 

 the posterior part of the lateral column of the opposite side, constituting 

 the crossed pyramidal tract; as it descends it gradually diminishes in size 

 as its fibers or their collaterals enter the gray matter of each successive 

 segment. 



2. A small portion, containing from 20 to 10 per cent, of the fibers, which 



does not decussate at the medulla but passes downward on the inner 

 side of the anterior column of the same side, constituting the direct 

 pyramidal tract or column Tiirck. This tract can be traced down, as 

 a rule, only as far as the mid-dorsal region. As it descends it becomes 

 smaller as its fibers cross the anterior commissure to enter the gray 

 matter of the opposite side. Thus all the fibers of the pyramidal tract 

 from each cerebral hemisphere eventually are brought into relation with 

 the cells of the gray matter of the opposite side of the cord. 



1 From the fact that the region included between the origin of these fibers and the internal 

 capsule presents somewhat the form of a pyramid with four sides, Charcot designated it the 

 pyramidal region and the fibers composing it the pyramidal tract. The base of the pyramid in- 

 cludes ^ the convolutions of the cortex in front of the Rolandic fissure. The summit of the 

 pyramid is truncated and covers the pyramidal region of the internal capsule. 



