THE BRA IX. 421 



lamina, into two parts, a superior and an inferior. The inferior part, 

 or that visible on the under surface of the brain, is called the "base" 

 of the crura cerebri. It consists of two conspicuous diverging bundles, 

 the fibres of which go to the corpora striata and internal capsule. The 

 superior, deep-seated portion of the crura cerebri is called the " teg- 

 mentum " or cap. Its fibres pass to the optic thalami and internal 

 capsule. 



The internal capsule accordingly represents, on each side, the con- 

 tinuation of the crus cerebri. But this continuity is an interrupted 

 onU The fibres forming the crus cerebri plunge, for the most part, 

 directly into the corpus striatum in front and the optic thalamus 

 behind, becoming dispersed in the gray substance of these ganglia, and, 

 to all appearance, terminating in or among their nerve cells. These 

 fibres, at the same time, are replaced by others which originate in the 

 cerebral ganglia, and which, passing obliquely upward and outward, 

 join the internal capsule, to continue their course toward the gray sub- 

 stance of the hemispheres. At the upper border of the ganglia, they 

 spread out in the diverging bundles of the corona radiata, and thus 

 reach, at last, the convolutions of the cortex. The internal capsule is 

 accordingly composed partly of fibres which come up from the crura 

 cerebri, and terminate in the cerebral ganglia, and partly of fibres 

 which start from the ganglia, and run upward to the cortex ; and the 

 communication between the cerebral convolutions and the spinal cord 

 is, for the greater part, an indirect communication through the cerebral 

 ganglia. 



Direct Medullary Fibres. Beside the fibres above described, there 

 i? evidence that the internal capsule contains also tracts of direct com- 

 munication, which pass through it from the convolutions to the crura 

 cerebri without interruption by the gray substance of the ganglia. 

 These direct fibres are of two kinds, namely ; first, motor fibres, pa- 

 from the convolutions about the fissure of Rolando, through the middle 

 part of the crura cerebri, to the pyramidal tracts of the spinal cord ; 

 and, secondly, sensitive fibres, passing from* the spinal cord along the 

 outer border of the crura cerebri, through the posterior part of the 

 internal capsule toward the convolutions of the occipital lobe. 



I. The direct motor fibres of the internal capsule have not been 

 clearly demonstrated by methods of dissection ; the intricate crossing 

 in the upper part of the capsule making it difficult to follow individual 

 fibres for a sufficient distance. Their existence is mainly inferred from 

 the occurrence of descending degenerations in this part of the brain. 

 According to Charcct,* destructive lesions of the cortical substance, 

 in the anterior and prsterior central convolutions, give rise to descend- 

 ing degenerations which pass through the internal capsule, crura 

 cerebri, anterior pyramids, and lateral columns of the cord. Such 



sur les Localisations dans les Maladies du Cerveau. Paris, 1878, pp. 

 156, 166. 



