656 PHYSIOLOGY. 



composed of white fibers. There is nothing in its appearance to let 

 anyone suppose that there are different tracts or bundles. However, 

 pathological anatomy, with its secondary degeneration, and embry- 

 ology, by reason of the myelin appearing in the bundles at different 

 stages of development of the foetus, reveal a number of segments per- 

 fectly separated either from a functional or pathological point of view. 

 The three bundles of fibers are distributed somewhat as follows 

 in the capsule : 



1. The Cortico-Pontal-Cerebellar Tract is composed of neuraxons 

 from the pyramidal cells of the frontal lobes. Then the neuraxons 

 pass through the anterior two-thirds of the anterior segment of the 

 internal capsule, then through the crusta, ending in some of the 

 pontal nuclei. These pontal nuclei are joined by neuraxons to the 

 fibers chiefly from half of the cerebellum of the opposite side by 

 the middle cerebellar penduncles, although some fibers are from the 

 cerebellar half of the same side. Hence the frontal lobes are ana- 

 tomically connected with the opposite cerebellar hemisphere. 



2. The Motor Tract arises from the neuraxons of the large pyrami- 

 dal cells of the ascending frontal and paracentral convolutions; then 

 goes through the anterior two-thirds of the posterior segment of the 

 internal capsule ; then through the crusta to the anterior pyramids of 

 the medulla oblongata, where they partly decussate, becoming the 

 crossed pyramidal tract of the opposite side of the spinal cord, and 

 ending in the cells of the anterior horns. Part of the motor tract 

 passes down on the side upon which it originated as the tract of Tlirck, 

 then through the anterior white commissure into the cells of the 

 anterior horn of the opposite side of the cord. Here we have a long 

 neuraxon or axon from the motor convolution to the anterior horns of 

 the opposite side of the spinal cord. From here a second axon starts 

 out to supply the muscles, making only two axons in the motor tract. 



The motor tract includes a band of fibers running from the 

 cortex to the nucleus of the various motor cranial nerves. Thus 

 the cortex sends motor fibers to the nucleus of the third, the fourth, 

 the motor division of the fifth, the sixth, the seventh, the motor 

 divisions of the ninth and tenth, and the eleventh and twelfth pairs. 

 We only know the cortical origin of the seventh, the motor branch of 

 the fifth, and the hypoglossal, and these originate from the lowest 

 third of the ascending frontal convolution; then they pass through 

 the knee, or genu, of the internal capsule and continue through the 

 crusta until they end in the nuclei of the various cranial motor nerves. 



