CENTRAL NERVOUS SYSTEM. 



245 



dogs, and by the studies of Melius 1 on secondary degenerations occurring in 

 the cord after very limited lesions of the motor cortex of monkeys. 



The direct pyramidal tracts are well marked only in man. They usually 

 disappear in the mid-thoracic region, having entered the gray substance by 

 way of the ventral commissure, in which they undergo decussation. The 

 crossed pyramidal tract shows the greatest diminution in area after passing 

 caudad of the cervical and lumbar enlargements respectively, and hence it is 

 inferred that the pyramidal fibres largely terminate at these levels of the 

 cord. 



Fig. 101.— Schema of the projection-fibres within the brain (Starr) : lateral view of the internal cap- 

 sule: A, tract from the frontal gyri to the pons nuclei, and so to the cerebellum ; />'. motor tract : I ', sen- 

 sory tract for touch (separated from B for the sake of clearness in the schema) ; />, visual tract ; E, audi- 

 tory tract; F, Q, II, superior, middle, and inferior cerebellar peduncles; J, fibres between the auditor; 

 nucleus and the inferior quadrigeminal body ; K, motor decussation in the bulb; 17, fourth ventricle. 

 The numerals refer to the cranial nerves. The sensory radiations k are seen to be massed toward the 

 occipital end of the hemisphere. 



Sherrington has put forward the view that the pyramidal fibres recross in 

 the cord, these recrossing fibres being derived in large part from a division 

 of the pyramidal fibres into two branches, one of which may cross fco the 

 opposite side of the cord, while the other continues its lirst course. Such 

 dividing fibres he designates as"geminal fibres;" and the number of them 

 is by no means small. 



The observations of Sherrington were made on monkeys (Macacus) aud 



1 Proceedings of the Royal Society, London, 1891 and 1895. 



