508 HANDBOOK OF PHYSIOLOGY. 



hippocampal sulcus: this fissure underlies the projection of the hippo- 

 campus major within the brain. There are three internal temporo-occi- 

 pital convolutions, of which the superior and inferior ones are usually 

 well marked, the middle one generally less so. 



The collateral fissure (corresponding to the eminentia collateralis) 

 forms the lower boundary of the superior temporo-occipital convolution. 



All the above details will be found indicated in the diagrams (Figs. 

 346, 347). 



Structure. The cerebrum is constructed, like the other chief divi- 

 sions of the cerebro-spinal system, of gray and white matter; and, as in 

 the case of the Cerebellum (and unlike the spinal cord and medulla ob- 

 longata), the gray matter (cortex) is external, and forms a capsule or 

 covering for the white substance. For the evident purpose of increasing 

 its amount without undue occupation of space, the gray matter is vari- 

 ously infolded so as to form the cerebral convolutions. 



The cortical gray matter of the brain consists of five layers (Mey- 

 nert) (Fig. 348). 



1. Superficial layer with abundance of neuroglia and a few small 

 multipolar ganglion-cells. 2. A large number of closely packed small 

 ganglion-cells of pyramidal shape. 3. The most important layer, and 

 the thickest of all: it contains many large pyramidal ganglion-cells, each 

 with a process running off from the apex vertically towards the free sur- 

 face, and lateral processes at the base which are always branched. Also 

 a median process from the base of each cell which is unbranched and 

 becomes continuous with the axis-cylinder of a nerve-fibre. 4. Numerous 

 ganglion-cells: termed the "granular formation" by Meynert. 5. 

 Spindle-shaped and branched ganglion-cells of moderate size arranged 

 chiefly parallel to the free surface (vide Fig. 348). 



According to recent observations by Bousfield, the fibres of the me- 

 dullary centre become connected with the multipolar ganglion-cells of 

 the fourth layer, and, from these latter, branches pass to the angles at 

 the bases of the pyramidal cells of the third layer of the cortex (Fig. 350, 

 a). From the apices of the pyramidal cells, the axis-cylinder processes 

 pass upwards for a considerable distance, and finally terminate in ovoid 

 corpuscles (Fig. 349), closely resembling, and homologous with, the cor- 

 puscles in which the ultimate ramifications of the branched cells of Pur- 

 kinje in the cerebellum terminate. Thus it would seem that the large 

 pyramidal cells of the third layer are themselves homologous with the 

 cells of Purkinje in the cerebellum. 



The white matter of the brain, as of the spinal cord, consists of 

 bundles of medullated, and, in the neighborhood of the gray matter, of 

 non-medullated nerve-fibres, which, however, as is the case in the cen- 

 tral nervous system generally, have no external nucleated nerve-sheath, 

 which are held together by delicate connective tissue. The size of the 

 fibres of the brain is usually less than that of the fibres of the spinal 

 cord: the average diameter of the former being about y^gir of an inch. 



