BRAIN AND NERVE 



103 



that is, the optic thalami. These two pairs of basal ganglia are 

 present in the brains of all vertebrates, and we are regarding 

 them here as parts of the " lower brain." The roofs and sides 

 of the lateral fore-brain vesicles are either not present at all as 

 nervous grey matter, or are imperfectly developed in the lower 

 vertebrates, and they become very important only in the 

 mammals. There they form a relatively thin sheet of gTey 

 matter which constitutes the superficial part of the hemispheres, 

 and this increases so greatly in area that it becomes crumpled 

 and folded in a complex way, so that it may be contained within 

 the cranial cavity. The crumpling leads to the formation of the 

 cerebral convolutions. 

 There is therefore a 

 thin layer of grey 

 matter — the cortex 

 cerebri — on the surface 

 of each hemisphere, 

 and beneath this there 

 are great bundles of 

 nerve fibres running 

 in various ways. Be- 

 neath these bundles of 

 white matter, again, 

 are the basal ganglia. 



The white matter of 

 the cerebral hemispheres form two main categories of bundles — 

 the commissural tracts and the projection tracts. We have 

 dealt with the latter in Fig. 28. The commissural tracts are 

 represented in Fig. 29. 



One system of commissural tracts, represented chiefly by the 

 corpus callosum, connect together the right and left halves of 

 the cerebrum. Probably corresponding parts of the two hemi- 

 spheres are joined by the fibres of the corpus callosum, but it is 

 also possible that any one part of one hemisphere is connected 

 with most parts of the other. In addition to these transverse 

 commissural tracts there is an elaborate system of internal 

 commissural or "' association " tracts in each hemisphere, and 

 some of these are represented in Fig. 29. They connect together 

 the various convolutions, or groups of convolutions, by short 

 paths, while longer tracts connect distant parts of the cortex 

 with each other. More and more, as cerebral physiology de_ 



Fig. 29. — A Section throuoh a Cerebrai. 

 Hemisphere near the Middle Plane to 

 SHOW the Main Commisural Tracts. 



