Si6 . TEXT-BOOK OF PHYSIOLOGY. 



From a study of this table it is apparent: (i) that some forms of sensor 

 impulses (those of pain and temperature sensibility) cross soon after their 

 entrance and pass up the opposite side of the cord; (2) that other forms of 

 sensor impulses (those of the sense of passive position and of movement and 

 tactile discrimination, Head) do not cross, but pass up on the same side as 

 the entering posterior nerve roots; (3) that tactile sensibility may or may not 

 be abolished on the side opposite the lesion; and (4) that the sense of cuta- 

 neous localization may be dissociated from the sense of passive position, and 

 remain intact when the latter is absent (Head). 



Encephalo-spinal or Motor Conduction. At birth the child is cap- 

 able of performing all the functions of organic life, such as sucking, swallow- 

 ing, 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 into 

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

 immature, but that their descending axons are incompletely invested with 

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

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

 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. 237.) At this point 

 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 



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 around the Rolandic fissure. The summit of the pyramid 

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



