628 THE NEKVOUS SYSTEM. 



In the lowest vertebrates the whole hemisphere is practically rhinencephalon. 

 Nevertheless, fibres coming from other parts of the nervous system and conveying 

 impressions from other sense organs than those of smell make their way into the 

 cerebral hemisphere and influence the state of its activities. In other words, the 

 hemisphere is primarily an olfactory receptive nucleus, but is also the place where 

 impressions of smell are brought under the modifying influences of other sensory 

 impressions before they make their effects manifest in behaviour. 



But it is only in the most highly organised types of brain, more especially 

 those of mammals and birds, that the non-olfactory senses acquire a representation 

 in the hemisphere which is relatively independent of, or at any rate not wholly 

 subservient to, the influence of the sense of smell. In the mammalian brain a 

 definite area of pallium is set apart to receive impressions of the tactile, visual, 

 acoustic, and other senses. This area is the neopallium. In the human brain it 

 has grown to such an extent that it forms almost the whole of the hemispheres 

 a mass far greater than the whole of the rest of the central nervous system. 



THE CEEEBEAL COMMISSUEES AND THE SEPTUM PELLUCIDUM. 



We have seen that certain fibres from the hippocampi cross from one hemisphere 

 to the other, using the upper part of the lamina terminalis as a bridge across the 

 median plane. But at an earlier stage of development other fibres can be detected 

 at a slightly lower level in the lamina terminalis forming a bundle, of oval outline 

 in sagittal section, called the commissura anterior. Its fibres come from the 

 olfactory bulb, area piriformis, tuberculum olfactorium, and a small temporal area 

 of neopallium. If the composition of the hippocampal commissure is analysed 

 in a foetus of the third month, it will be found that there are intermingled with 

 the truly hippocampal fibres some which come from the neopallium. During the 

 fourth month the bulk of the neopallial element in this dorsal commissure 

 outgrows the hippocampal element. The latter fibres become crowded into the 

 postero-inferior corner of the commissure and the neopallial fibres come to form a 

 flattened transverse bridge the corpus callosum above them. These fibres are en- 

 closed in a neuroglial matrix derived from the lamina terminalis and the adjoining 

 paraterminal bodies. Some nerve-cells also may make their way into this matrix. 

 As it elongates, the corpus callosum pushes its way forwards in the upper part of 

 the paraterminal body of each hemisphere, and as development proceeds a small area 

 of this body becomes almost completely circumscribed by the corpus callosum and 

 commissura hippocampi. As these commissural bands increase in size this small 

 circumscribed patch of paraterminal body becomes greatly stretched and ex- 

 panded to form a thin translucent leaf. The two leaves thus formed in the medial 

 walls of the two hemispheres are known as the septum pellucidum ; and the narrow 

 cleft that separates them the one from the other in the median plane is called the 

 cavum septi pellucidi. 



There is still an element of uncertainty concerning the precise manner in which 

 these changes are brought about, and especially as to the precise mode of closure 

 of the cavum septi. As the cerebral hemisphere expands, some parts of it grow 

 forwards, others upwards, and others again backwards. Such growth in each 

 part will naturally tend to exert traction upon its commissural fibres that pass 

 through the corpus callosum. Hence the anterior part of this great commissure 

 becomes drawn forwards, its posterior part backwards, and the greater intermediate 

 part upwards; so that it comes to assume the form shown in Fig. 557, C. 

 As the posterior part of the corpus callosum pushes its way backwards, it exerts 

 traction upon the fibres of the hippocampal commissure and their matrix, which 

 becomes enormously stretched so as to form a thin lamella (the floor of the cavum 

 septi) stretching from a point just above the anterior commissure to the under 

 surface of the swollen posterior end of the corpus callosum, which is called the 

 splenium (Fig. 558). The hippocampal commissural fibres are scattered through- 

 out this lamella. The backward growth of the splenium also thrusts back th( 



