THE CEREBRAL HEMISPHERES. 691 



of the fibres with an exactitude that seems hardly justified, in all instances, by the facts, 

 in view of the inevitable difficulty and uncertainty of some of the processes employed; 

 and the graphic and admirable delineations by which the work is illustrated, though pro- 

 fessedly schematic, present a degree of ideality which inspires some distrust with regard 

 to the accuracy of the general conclusions. According to Luys, the fibres of the en- 

 cephalon have several directions, as follows : 



The gray matter of the cerebral hemispheres, as we shall see farther on, is composed 

 of a mass of nerve-cells, connected together by their prolongations into a plexus, which, 

 in its turn, is connected with the fibres of the white substance. From this cortical cellu- 

 lar plexus, white fibres arise, which may be divided, according to their direction and 

 destination, into two classes : The first class consists of curved commissural fibres, which 

 pass into the white substance to a certain depth and return to the gray matter, connect- 

 ing thus the gray substance of adjacent convolutions. The existence of these fibres 

 and their direction are well established. The second class consists of fibres which, 

 arising from the gray substance of the convolutions, connect these with the corpora 

 striata and the optic thalami. These may be called the converging fibres ; and their 

 general direction, as far as it has been ascertained, is shown in figure 226. 



Arising from the internal, concave surface of the cortical substance of the cerebrum, 

 the converging fibres, at first running side by side with the curved commissural fibres, 

 separate from the latter as they curve backward to pass again to the cortical substance, 

 and are directed toward the corpora striata and the optic thalami. The limits of the 

 irregular planes of separation of the commissural and the converging fibres contribute to 

 form the boundaries of the ventricular cavities of the brain. If we study the course of 

 the converging fibres arising from all points in the concave surface of the cerebral gray 

 matter, we find that they take various directions. The fibres from the anterior region of 

 the cerebrum pass backward and form distinct fasciculi which converge to the gray sub- 

 stance of the corpora striata. The fibres from the middle portion converge regularly to 

 the middle region of the external portions of the optic thalami. The fibres from the pos- 

 terior portion pass from behind forward and distribute themselves in the posterior portion 

 of the optic thalami. The fibres from the convolutions of the hippocampi and the fascia 

 dentata are lost in the gray substance lining the internal borders of the optic thalami. In 

 addition to these converging fibres and the curved commissural fibres connecting the 

 different convolutions of each hemisphere with each other, are commissural fibres which 

 connect the two hemispheres, as well as fibres connecting together the corpora striata 

 and the optic thalami of the two sides. 



Certain of the fibres converging from the gray substance of the hemispheres to the 

 corpora striata and optic thalami are probably connected with the cells in the gray mat- 

 ter of these parts. Other fibres pass through the corpora striata and optic thalami to 

 become finally connected with the fibres of the medulla oblougata, and, through the 

 medulla oblongata, with the columns of the spinal cord. Following the antero-lateral 

 columns of the cord from below upward, they ascend to the medulla oblongata, decussate 

 in the median line, and pass from the medulla to the brain. Certain of these ascending 

 fibres, which are nearly all continuations of the antero-lateral columns of the cord, ascend 

 to the brain by passing deeply through the pons Varolii ; other fibres ascend in the 

 cerebral peduncles, or crura cerebri ; and other fibres pass to the tubercula quadrigemina. 

 As the bundles of fibres ascend from the medulla oblongata, they increase in number by 

 reinforcements of fibres, probably derived from the cells of the collections of gray mat- 

 ter in their course. 



The Cerebral Convolutions. 



The cerebrum, as we have already stated, constitutes more than four-fifths of the en- 

 cephalic mass. Its surface is marked by fissures and convolutions, which latter serve 



