THE PHYSIOLOGY OF THE BRAIN. 619 



foramen of Monro, fM, with the third ventricle is the left 

 lateral ventricle, 2, bounded on the inner side by the thin 

 septum lucidum. Between the septa lucida is the fifth ven- 

 tricle, 5. The gray mass, JVc, to the side of the lateral ven- 

 tricle is the caudate nucleus and the mass Ln the lenticular 

 nucleus of the corpus striatum. In front and at a level dif- 

 erent from that of the diagram the two are continuous. 



The band of white fibres, ic, lying here between the 

 lenticular nucleus on the outer side and the caudate nucleus 

 .and optic thalamus on the inner side is the internal capsule : 

 ec is the external capsule. Fl is the cortical gray matter of 

 tH frontal lobe of the cerebrum; PI, of the parietal lobe; 

 Oc.l. of the occipital lobe: Ro, the fissure of Rolando; Po, the 

 parieto-occipital fissure. The course of many fibres in the 

 forebrain is still uncertain, but some important paths have 

 been traced by anatomical and microscopic work, and still 

 more by following tracts of degeneration resulting from cer- 

 tain lesions, as in the case of the spinal cord ; and also by 

 noticing the results of stimulation or removal of definite areas. 



Taking first the pes of the crus cerebri (Fig. 176), which 

 consists entirely of longitudinal fibres, we find that the py- 

 ramidal tract, py, Fig. 177, is continued through the internal 

 capsule and radiates beyond it, to end in the cortex of the 

 frontal and parietal lobes in the region of the fissure of Rolando. 

 These fibres are all efferent and degenerate to their endings in 

 the gray matter of the cord or the motor nuclei of cranial nerves 

 when the cortex in the Rolandic region is removed. A second 

 collection of fibres in the pes is the frontal, and its fibres, /r, 

 can be traced to the frontal region of the cortex; when that 

 is removed the fibres degenerate as far as the gray matter of 

 the pons, from which they are probably connected by other 

 fibres with the opposite side of the cerebellum. A third set 

 of fibres in the pes is the temporo-occipital, oc: they also pass 

 through the internal capsule to the corresponding region of 

 the cortex: they can be traced as far as the gray matter of 

 the pons, and appear to be fibres of descending degeneration. 

 Another set of fibres, ca, of descending degeneration in the 

 internal capsule has no immediate connection with the cortex: 

 it arises in the caudate nucleus ; the course of the fibres be- 

 yond the pons is not known. The lenticular nucleus also 

 gives off fibres, #, to the internal capsule, which probably con- 

 nect the corpus striatum through the pes with the pons and 



