688 FUNCTIONS OF THE CER EBRO -SPIN AL NERVOUS SYSTEM. 



in old age, the convolutions are simpler and have fewer undulations, and the 

 thickness of their cortical substance is much inferior; and the same is true 

 of the adult brain of some of the least cultivated races of mankind. The 

 structure of the cortical substance has been most carefully investigated by 

 Mr. Lockhart Clarke. 1 The convolutions present on section at least seven 

 layers of nervous substance, the concentric arrangement of which is most 

 conspicuous at the extremity of the posterior lobe, and is rendered apparent 

 by a slight difference in hue. In this situation all the nerve-cells are small, 

 but differ considerably in shape, and are much more abundant in some layers 

 than in others. In the superficial layer, which is pale, they are round, oval, 

 fusiform, or angular, but are not numerous. The second layer is darker, and 

 is densely crowded with cells of similar form and size, mingled with others that 

 are pyriform and lie with their tapering ends either towards the surface or 

 parallel with it, in connection with fibres which run in corresponding direc- 

 tions. The broader ends of the pyramidal cells give off two or more pro- 



FIG. 243. 



Diagram of the mutual relations of the principal Encenhalic centres, as shown in a vertical section : 

 A, (Vri-bnini ; it, Cerebellum ; c, .Sensori-motor tract, including the Olfactive ganglion nil', the Optic opt, 

 and the Auditory and, with the Thalaini Optici I/ml, and the Corpora St.riata cs ; D, Medulla Ohlon.sjata; 

 K, Spinal Cord; a, olfactive nerve; 6, optic; c, auditory ; </. jineunio^astric ; e, hypoglossal ; /, spinal : 

 fibres of the medullary substance of the Cerebrum are shown, connecting its ganglionic surface with 

 the Sensori-motor tract. 



cesses which run partly towards the central white axis of the convolutions, 

 and partly horizontally along the plane of the layer. The third layer is of 

 a much paler color. It is crossed at right angles by narrow and elongated 

 groups of small cells and nuclei, intermediate to which are bundles of fibres 

 radiating towards the surface from the central white axis of the convolutions. 



1 Sec 1'roeced. of Roy Soc., vol. xii, No. 57. :i nd Dr. Maudsley'.s Physiology and 

 Pathology of the Mind, 18(18, p. 00, from which last tin- account in the t'ext U taken. 

 A i;ood description of the structure of tho brain is also ^iven by Arndt in Schultze's 

 Archiv, IM. iii, p. 4-41, 18U7. See also Cleland, C^iiart. Jotini. of Microscop. Sci., 

 1870, p. 12H ; and Meynert, in Strieker's Human and Comp Histology, vol. ii, p. 382 ; 

 Gerlnch, Ccntralblatt, 1872, p. 273 ; Kindfleisch, Ccnlriilblatt, 1872, p. 277. 



