314 / THE BRAIN. 



spinal cord may be gathered by considering that a bifurcation of the 

 fibres takes place in the medulla oblongata, and upon one of the re- 

 sulting bundles, the eras cerebri, the cerebrum is formed, on the other 

 the cerebellum. The crus cerebri is composed of three strands : an infe- 

 rior, the fibres of which have come from the anterior pyramids, and in 

 part from the olivary bodies. This strand ends in the corpus striatum, 

 its fibres not, however, blending abruptly with the vesicular matter, but 

 passing into it in bundles. It is essentially motor. A superior, which 

 is derived from the posterior pyramids, and terminates in the thalamus. 

 It is essentially sensory. Between these, constituting the third portion 

 strand it can scarcely with propriety be called is a layer of dark vesic- 

 ular material, the locus niger. It is to be understood that the motor 

 strands of the opposite sides decussate in the medulla oblongata ; the 

 sensory strands decussate in the mesocephalon. 



The other bundle, arising in the original bifurcation, assumes the des- 

 Formation of ignation of crus cerebelli. On it the cerebellum is devel- 

 the cerebellum. O p Q ^ t It consists essentially of fibres from the restiform 

 bodies, re-enforced by others which have come from the anterior pyramids 

 under the name of arciform fibres. These together make their way to 

 the interior ganglion of the cerebellum, the corpus dentatum, and there 

 they end. But the crus cerebelli contains likewise two other great 

 strands : an inferior, which constitutes the commissure of the two cere- 

 bellar hemispheres, and which, running round the entire prolongations of 

 the spinal cord, forms the pons varolii ; a superior, the processus cere- 

 belli ad testes, which unites the cerebellum and cerebrum. 



Of the portions of the spinal cord on which the cerebrum is to be de- 

 veloped, those which are sensory end in the optic thalamus, those which 

 are motor in the corpus striatum. The thalamus and striatum of each 

 side may be regarded as one compound ganglion, since, like the columns 

 of the cord, they are united by a gray and a white commissure. Of the 

 portions on which the cerebellum is to be developed, the termination is 

 in the central ganglion of the cerebellum, the corpus dentatum. 



At the place of bifurcation of the constituent strands of the crus cere- 

 bri and crus cerebelli from each other in the medulla oblon- 



" ar} y ' gata, there is intercalated or included a ganglion, which, with 

 its apparatus, constitutes the olivary body, the fibres of which make 

 their way upward between the two preceding bundles, and, having bi- 

 furcated, one branch goes to the quadrigemina and the other to the op- 

 tic thalamus, the latter constituting, as has been said, a part of the 

 crus cerebri. The seat of power of the medulla oblongata is in this 

 ganglion. 



Such being the anatomical construction of the crus cerebri, it may be 

 physiologically regarded as a compound- strand, the anterior portion of 



