248 PHYSIOLOGY FOR DENTAL STl'DKNTs. 



occurs in the higher animals. The afferent impulse when it en- 

 ters the cord is more likely to travel up the posterior columns 

 and then, as already outlined, to the cerebrum, when' it ends on 

 the large pyramidal nerve cells of the grey matter. 



From the pyramidal cells spring the fibers of the pyr<nni<l<tl 

 tracts, which, as they pass downward through the white matter of 

 the cerebrum, crowd closer and closer together until, by the time 

 the basal ganglia are reached (optic thalamus on the inside, 

 and corpus striatum on the outside), they form a narrow bun- 

 dle which occupies the middle portion of the strip of white mat- 

 ter which lies between these ganglia. This white matter is 

 called the internal capsule (Fig. 46), and it is of very great 

 clinical interest because, being in the neighborhood of a large 

 artery (branch of middle cerebral), which sometimes bursts in 

 elderly people, it is apt to become torn up by extravasated 

 blood, thus destroying the pyramidal fibers and causing paraly- 

 sis. This is what occurs in apoplexy. Below the internal capsnK 

 the fibers run into the crura cerebri, then into the pons, thence 

 into the medulla oblongata, in the front of which they form a dis- 

 tinct bulging called the pyramid; hence their name pyramidal 

 fibers (see Fig. 45). 



In the lower portion of the medulla, a most interesting thing 

 occurs, namely, three-fourths of the fibers cross to the oppo- 

 site side, thus constituting the dccussation of the pijrtitni>lx 

 (Fig. 44). These crossed fibers run down in the lateral columns 

 of the spinal cord as the crossed pyramidal tracts. The pyra- 

 midal fibers which do not cross in the medulla form the direct 

 pyramidal tracts of the cord, and they gradually cross in tin- 

 cord itself. The pyramidal fibers end by synapsis around the 

 cells of the anterior horn, so that all fibers from the cerebrum 

 ultimately cross to the opposite side before they reach the anterior 

 horn cells, for which reason it happens that a lesion involving 

 the pyramidal tract anywhere above the decussation, such as the 

 haemorrhage in the internal capsule above referred to. nhvays 

 causes paralysis of the opposlit \/Vc of the body (hemiplegia). 



These facts regarding the course of the pyramidal fibers ha\v 

 been ascertained by microscopic examination of sections from 



