STRUCTURE OF THE BRAIN. 125 



peduncles of the brain, which are, as it were, the roots of that 

 organ uniting it to the .other parts of the encephalon; the 

 pons Varolii, or annular protuberance; the medulla oblongata; 

 and the origins of the cranial nerves. 



The brain, as well as all the divisions of the cerebro- 

 spinal nervous centre, is composed of two distinct sub- 

 stances : the gray or cortical substance, the first on account of 

 its colour, and the second because it is on the surface of the 

 brain, like a bark (cortex); and the white substance, which is 

 everywhere completely surrounded by the gray. 



The gray substance is pulpy and less consistent than the 

 white. It is disposed around the principal organs of the ence- 

 phalon in a superficial layer, and enters into their substance 

 in masses varying in form and volume. The white substance 

 is filamentous in texture, and exhibits, according to the region, 

 bundles, cords, or layers, composed of slender fibres, which 

 are alike in the nervous centres and in all the nerves of the 

 body. The white greatly exceeds the gray substance in total 

 amount. 



On laying open the brain we find it is composed of a 

 central nucleus or knot unique and symmetrical, a sort of 

 terminal enlargement of the nervous axis, and of two hemi- 

 spheres united by this central portion, of which they are, as 

 it were, a sort of double expansion. This central nucleus 

 comprises parts very complicated in their structure and in 

 their relative positions. The principal ones are the thalamus 

 opticus'Qi optic bed, the corpora striata or striated bodies, and 

 the corpus callosum or hard body. All these subdivisions of 

 the cerebral centre are united to each other, to the peduncles, 

 and to the hemispheres. Thus the corpus callosum, which 

 serves as an envelope of the cerebral centre, receives fibres 

 from the peduncles and from the optic bed, prolongs its 

 edges into the substance of the hemispheres, between which 

 it is, as already stated, the principal means of union. 



In the substance of the cerebral centre there are three 

 cavities, called the ventricles of the brain; two are lateral, 

 and the third or middle is placed on the median line; they 

 communicate with each other, and are bathed with a serous 

 fluid analogous to that which lubricates the spinal cord. On 



