THE ARRANGEMENT OF THE FIBRES OF THE BRAIN. 627 



There are also other collections of gray matter in the central white mat- 

 ter of the cerebellum, one of which, called the "nucleus of the roof," is con- 

 nected with the two inferior peduncles. 



THE ARRANGEMENT OF THE FIBRES OF THE BRAIN. 



544. The systems, tracts, and bundles of fibres in which the white 

 matter of the brain is arranged, may be distinguished from each other, partly 

 through mere mechanical separation by means of the scalpel, partly by being 

 traced out with the help of the microscope, but, as in the spinal cord, much 

 more fully and completely by differences of development, and by the method 

 of degeneration. 



FIG. 145. 



Outline of a Sagittal Section through the Hemisphere Man. (Sherrington.) The section is 

 taken not far to the right of the median plane and is one-half linear or natural size. The gray 

 matter of the corpus striatnm and thalamus is shaded. Nc, Nc, the caudate nucleus ; Pt, the 

 putamen, and Gp, the globus pallidus of the lenticular nucleus; T, the optic thalamus; CI, the 

 internal capsule with a streaked appearance revealing approximately the direction taken by fibre- 

 bundles passing into it from the portion of corona radiata over it. In these sets of bundles may 

 be broadly distinguished a frontal system, fron, a pyramidal system, P Y (subdivisible into cranial 

 (cran.), brachial (brack.), dorso-lumbar (dors, lum.), and lumbo-sacral (lum. sac.) parts, and a tern- 

 poro-occipital system, sens.; the situation of the genu of the internal capsule is indicated by g. 

 CR, the crus cerebri ; Oc, the so-called optic radiations passing into the occipital lobe; cc, the 

 splenial end of the corpus callosum ; v, v, v. the lateral ventricle cut across in three different 

 places ; F, the fornix in cross-section ; Op, the optic tract in cross-section. Part of the cerebellum 

 is seen in outline to the right. 



We have seen that a marked feature of the brain is presented by the two 

 crura cerebri which, running forward from the hind parts of the brain, 

 spread out into each cerebral hemisphere. We have also seen that the crus 

 in the wide sense of the word consists of two parts, a dorsal part, the teg- 

 mentum, and a ventral part, the pes or crusta, and that these two parts differ 

 very strikingly from each other in structure and in relations. The pes con- 

 sists exclusively of bundles of longitudinal fibres, and we may trace these 

 from the cerebral hemispheres into the pons and some of them beyond the 

 pons into the bulb and spinal cord. The tegmentum is more complex in 

 structure ; it consists of gray matter, and of fibres and bundles of fibres 

 having various relations both with the collections of gray matter lying 

 within itself and with surrounding structures. It too has connections with 

 the parts lying in front of it, and with the parts lying behind it ; we may 

 trace it, too, backward through the pons into the bulb and forward to the 

 optic thalamus. If we allow ourselves to conceive of the optic thalamus as 



