70 TUF. NERVOUS S1STKM AM) ITS < '< >NSKKVATION 



The cerebrum itself and the ccrd iclluiu present a con- 

 trast to the other portions of the 1. rain when studied with 

 regard to the distribution of gray and white matter. In 

 them there is a surface layer of gray substance, while the in- 

 terior and by far the larger part of the whole ma- 

 is made up, in the main, of multitudinous fibers. The 

 surface layer is called the cortex, the primitive meaning 

 of the word being the bark of a tree. Certain submerged 

 portions of gray matter are found in the midst of the 

 internal white matter in both the cerebrum and the 

 cerebellum. 



The human cerebrum consists of two hemispheres, 

 between which there is a very deep longitudinal lissure. 

 The separation of the two halves is so marked that one 

 thinks of the walnut kernel with its comparatively slender 

 i-lhmus. The comparison is faulty, however, in that the 

 cerebral hemispheres have more than one bond of union. 

 Their principal tie is to be found at the bottom of the 

 longitudinal fissure, and when cut it appears in section as 

 a white arch. It is the corpus callosum, a structure to 

 which the physiologists of the seventeen! h century ascribed 

 much importance. Several minor connections are ob- 

 served lower down, all adjacent to the third ventricle. 



In all the higher animals the surface of the cerebrum 

 is much scored by fissures. These are called *///</. and the 

 convex ridge- between them are spoken of as <///// or con- 

 volutions. Such a type of organization evidently provides 

 for a much more extensive area of cortex than would exist 

 otherwise. Since cortical development stands in some 

 relation to the intelligence and general rank of theanimal 

 in the scale of being, one is disposed to look for an espe- 

 cially marked wrinkling in the case of man. The human 

 brain is indeed, much more fissured than that of the 

 ralibit, but it i- not much more so than that of the cow. 

 The attempt to make a great deal of this feature yields but 

 little comfort (Fig. i:i). 



1 legions of the surface of the cerebrum are usually 

 denoted by employing the names of tho-e cranial bones 



