614 



HANDBOOK OF PHYSIOLOGY. 



pes. Separating these two parts, is a mass of gray matter of the shape 

 of a lens, called the locus or nucleus niger or substantia nigra. 



The tegmentnm situated dorsally ends for the most part in the 

 neighborhood of the optic thalamus and the parts beneath. In conse- 

 quence of this the fibres of the pes are allowed to come dorsally and to 

 proceed between the optic thalamus and the more posterior part (the 

 lenticular nucleus) of the corpus striatum, on their course to the cere- 

 bral cortex. When in this situation they form a compact mass of fibres. 

 As they pass more dorsally the fibres spread out in the form of a fan, 

 and this arrangement is called the corona radiata. The fibres of the pes 



Fig. 369. Diagram of the motor tract as shown in a diagrammatic horizontal section 

 through the cerebral hemispheres, Crura, Pons, and Medulla. Fr. , Frontal lobe; Oc., occipital 

 lobe; AF. , ascending frontal, AP. , ascending parietal convolutions; PCF. , pre-central fissure, 

 in front of the ascending frontal convolution; FR. , fissure of Rolando; IPF., inter-parietal fis- 

 sure, a section of crus is lettered on the left side. SN. , Substantia nigra ; Py . , pyramidal motor 

 fibre, which on the right is shown as continuous lines converging to pass through the posterior 

 limb of 1C. internal capsule (the knee or elbow of which is shown thus *) upward into the 

 hemisphere and downward through the pons to cross the medulla in the anterior pyramids. 

 (Gowers.) 



are found to stretch not only between the optic thalamus and the len- 

 ticular nucleus, but also more anteriorly between the former and the 

 caudate nucleus of the corpus striatum which, as we have seen, is to be 

 seen in the floor of the lateral ventricle. The fibres of the pes thus 

 spread out, have the form of a fan bent upon itself as they rise to pass 

 into the cerebral hemisphere. This constitutes the internal capsule, and 

 that portion of it which forms the angle at which the fibres are bent i& 

 called the genu of the capsule, that in front of it being the front, and 

 that behind, the hind limb. The fibres constituting the internal cap- 

 sule are distributed to different districts of the cerebral cortex. They 

 are made up of fibres not only constituting the pyramidal system, but 



