254 ANATOMY OF THE OENTEAL NEKVOUS SYSTEM. 



Hemiplegias from cortical lesions are very rare. Hemiplegias that proceed from the- 

 midbrain or from deeper-lying points are still more rare, and are mostly associated 

 with symptoms involving the cranial nerves, which indicate the seat of the lesion. 



On the other hand, both anatomical considerations and clinical experience teach 

 us that cerebral affections involving single parts of the body — for example, a, hand — ■ 

 are only very rarely produced by lesions in the internal capsule.. This is, indeed, 

 because the fibers are so closely crowded together there that a lesion can hardly in- 

 volve separate bundles of fibers alone without involving those near by. Monoplegias 

 and monospasms not infrequently arise from cortical lesions, however. There a 

 lesion may even be of relatively large size before it involves a neighboring center. 

 The accompanying diagram (Fig. 164) will serve to elucidate what has been said. 

 It shows why monoplegias proceed more frequently from the cortex and hemiplegias 

 more frequently from deeper-lying parts of the brain; for it is at once seen that a 

 lesion of given extent located in the cortex may easily involve one center alone, 

 A\-hereas a similar lesion situated farther below will involve the fibers of many centers. 



It has not as yet been learned what symptoms appear when fibers of association- 

 bundles alone are involved, on account of the proximity of these fibers to the corona 

 radiata. 



Probably certain forms of disturbances of speech, reading, and hearing belong in 

 this category. Furthermore, little is known concerning the symptoms appearing after 

 a loss of function (Fiiiictiuiisaiisfall) of the corpus callosum. It appears that under 

 certain conditions it may be entirely destroyed without the appearance of disturb- 

 ances of motility, of co-ordination, of sensibility, of the reflexes, of the special senses, 

 or of speech, and without the manifestation of any considerable disturbance of the 

 intellect. Uncertain gait, without actual vertigo or ataxia, was once observed in a 

 case of disease of the corpus callosum. 



The fibers of the corona radiata terminate, in great part, therefore, in 

 the interbrain, in the thalamus opticus. The other fibers pass on farther 

 downward and backward in the internal capsule. They thus come to lie 

 free, for the most part, on the under surface of the brain, behind the thala- 

 mus. These thick, white bundles there emerging from the brain-mass are 

 designated as the foot of the cms cerebri, pes peduncuU, or crusta (Fig. 165, 

 below and to left). 



As is seen in the accompanying frontal section, this free part of the 

 internal capsule, the fibers of which curve caudad as the cms cerebri, lies 

 ventral to the thalamus. Into this foot, the pes pedunculi of the eras cerebri, 

 pass the bundles of the frontal tract to the pons, those of the temporal tract 

 to the pons, and those of the pyramidal tract. The coronal fibers of the 

 opticus and the tegmental radiation do not enter the pes. Farther caudad, 

 in the region of the corpora quadrigemina, the nerve-fibers which come 

 from the thalamus and from other brain-parts, also those from the tegmental 

 radiation, lie dorsal to the pes in a position corresponding to that of the 

 thalamus. The fibers from the forebrain, the interbrain, and the midbrain 

 are there divided into a ventral part, the foot, pes, or crusta, — and a dorsal 

 part, the tegmentum. 



