646 THE NEEVOUS SYSTEM. 



any one sense, but as the place of meeting (and the physical counterpart of the 

 blending in consciousness) of the impressions of different senses. In the human 

 brain the neopallium becomes mapped out into a large series (more than forty) of 

 areas, which differ one from the other in structure and in their connexions, and 

 presumably therefore in their functions ; and many of these areas may be further 

 subdivided into a series of less obtrusively differentiated territories (Figs. 553 

 and 581). 



The gray matter of the neopallium is spread over the surface of the white 

 matter as a thin film (cortex cerebri), which is nowhere more than 4 millimetres, 

 and may be only T25 millimetres thick. In different regions it presents every 

 gradation of thickness between these two extremes. As the cortex increases in 

 volume it does so not by any addition to its depth, but solely by an expansion of 

 its superficial area. Thus it happens that in all larger mammalian brains, as the 

 cerebral hemisphere expands and there is an increasing disproportion between the 

 bulk of the hemisphere and the area of its surface, the cortex must become folded 

 to accommodate itself to the limited area of surface upon which it has to be packed. 

 But this process of folding does not take place in any haphazard or purely 

 mechanical way. The situations of the furrows or sulci which make their 

 appearance are determined, for the most part, by the arrangement and the 

 relative rates of expansion of the various areas into which the neopallium becomes 

 differentiated. 



The great majority of the furrows belong to a group, which we may call (1) 

 sulci terminales, i.e. they make their appearance along the boundary lines between 

 areas of different structure. The fissura rhinalis and sulcus centralis are examples 

 of this group. Another group, which may be called (2) sulci axiales, develop by 

 the folding of areas of uniform structure, i.e. along the axis of certain territories. 

 The retro-calcarine sulcus and the sulcus occipitalis lateralis belong to this 

 group. There is a third group of (3) sulci operculati, where the edge of one area 

 becomes pushed over an adjoining territory, so that a trough is formed (Fig. 575, C), 

 which is neither a limiting nor an axial sulcus. The sulcus lunatus is an example. 

 And finally there is a fourth group, in which some more definitely mechanical 

 factor comes into play to complicate the operation of these other factors, or even 

 to determine the development of a furrow. The sulcus parieto-occipitalis and 

 the fissura lateralis are examples of the fourth group. 



[It is the custom to call certain furrows sulci and others fissures, and to call 

 some of them complete, because they indent the whole thickness of the wall of the 

 ventricle, and to call the rest incomplete. There is no justification whatever for any 

 such distinctions. 



It is usual also to subdivide the surface of the hemisphere in a purely arbitrary 

 manner into "lobes" and to speak of interlolar fissures, but this is an artificial and 

 misleading terminology which we shall avoid as far as possible.] 



Fissura Longitudinalis Cerebri. The longitudinal fissure is not a fissure 

 of the cortex but is the great cleft between the two cerebral hemispheres. In 

 front and behind it separates the cerebral hemispheres completely the one from 

 the other. In its middle part, however, the fissure is interrupted and floored 

 by the corpus callosum, a white commissural band, which passes between the 

 hemispheres and connects them together. The superior surface of the corpus 

 callosum is displayed when the contiguous medial surfaces of the cerebral 

 hemispheres are drawn asunder. The longitudinal fissure is occupied by a median 

 fold of dura mater, termed the falx cerebri, which partially subdivides the part 

 of the cranial cavity allotted to the cerebrum into a right and left chamber. 



External Configuration of each Cerebral Hemisphere. Each cerebral hemi- 

 sphere presents a lateral, a medial, and an inferior surface. The lateral surface is 

 convex and is adapted accurately to the internal surface of the cranial vault. The 

 medial surface is flat and perpendicular, and bounds the longitudinal fissure. In 

 great part it is separated from the corresponding surface of the opposite hemisphere 

 by the falx cerebri. The inferior surface is irregular and is adapted to the 

 anterior and middle cranial fossae of the cranial floor and, behind these, to the 

 superior surface of the tentorium cerebelli. Traversing this surface in a transverse 



