232 THE NERVOUS SYSTEM 



as well as the septum pellucidum. The portion of the lamina terminalis which 

 enters into the formation of the septum becomes hollow as a result of the stretch- 

 ing to which it is subjected, and the resulting cavity is known as the cavum septi 

 pellucidi. 



The cerebral hemispheres are incompletely separated from each other by 

 the longitudinal fissure of the cerebrum, at the bottom of which lies a broad band 

 of commissural fibers, the corpus callosum, which forms the chief bond of union 

 between them. Each hemisphere has three surfaces: a convex dorsolateral 

 surface (Fig. 166), a median surface flattened against the opposite hemisphere 

 (Fig. 170), and a very irregular ventral or basal surface. A dorsal border sepa- 

 rates the dorsolateral from the medial surface; and a lateral border marks the 

 transition between the dorsolateral and basal surfaces. One may recognize 

 also frontal, occipital, and temporal poles (Fig. 166). The long axis of the hemi- 

 sphere extends between the frontal and occipital poles, and in man is placed 

 almost at right angles to the long axis of the body (Fig. 33) ; while in other mam- 

 mals it corresponds more nearly to the body axis. On this account it will be 

 convenient in the description of the human cerebral hemisphere to take the 

 occiput as a point of reference and use the term "posterior" in place of "caudal." 

 Otherwise our directive terms remain the same rostral, dorsal, and ventral 

 except that for the term "ventral" we shall often use the word "basal." 



The cerebral cortex is a layer of gray matter spread over the surface of the 

 hemisphere; and its area is greatly increased by the occurrence of folds or gyri 

 separated by deep sulci. That part of the cortex which belongs to the rhinen- 

 cephalon and is phylogenetically the oldest is designated as the archipallium. 

 It is separated from the newer and in mammals much larger neopallium or non- 

 olfactory cortex by the rhinal fissure (Figs. 83, 171). 



The Neopallium. The development of the neopallium is so much greater 

 in man than in the sheep, and the arrangement of the gyri and sulci is so dif- 

 ferent in the two forms that but little can be learned by a cursory comparison of 

 these structures in the two brains. We shall, accordingly, confine our atten- 

 tion almost exclusively to the arrangement of the neopallium in man. 



THE DORSOLATERAL SURFACE OF THE HEMISPHERE 



By means of some of the more important sulci the cortex is marked off into 

 well-defined areas, known as the frontal, parietal, temporal, and occipital lobes 

 (Fig. 167). To these should be added a lobe buried at the bottom of the lateral 

 fissure and known as the insula (Fig. 169). In the delimitation of these lobes 



