THE ISTHMUS. 777 



the fourth ventricle. After passing through the pons Varolii and the upper stage 

 of the crura cerebri, they enter the thalami optici. 



To the longitudinal fibres which continue those of the spinal cord, must be 

 added the fibres of the corpora restiformia and those of the anterior cerebellar 

 peduncles. 



The corpora restiformia proceed from the cerebellum, of which they form the 

 posterior peduncles, and go to be lost on the surface or in the mass of the 

 medulla oblongata, under the surface of the white transverse tractus. 



The anterior cerebellar peduncles descend from the cerebellum, approach the 

 middle line, concur in the formation of the upper stage of the crura cerebri, and 

 terminate in the thalami optici. 



To this important system of white longitudinal fibres — a prolongation of those 

 of the spinal cord — are found annexed as complimentary elements in the organi- 

 zation of the isthmus, several systems of transverse fibres and masses of grey 

 substance. The following is a summary account of the arrangement of these 

 new elements. 



In proceeding from behind to before, we notice, among the ivhite transverse 

 fibres — 



1. The expansion of arciform fibres which sometimes covers the inferior face 

 of the medulla oblongata (Fig. 456, y) : their superior extremity is lost on the 

 corpus restiforme ; the inferior passes into the intermediate fissure of the pyramid 

 and the lateral fasciculus. It is derived from the column of GoU (Fere). 



2. The proper fibres of the pons Varolii : they constitute a very thick semi- 

 circular fasciculus, the extremities of which fonn the middle cerebellar peduncles 

 and enter the cerebellum. A portion of this fasciculus envelops, inferiorly and 

 laterally, the longitudinal fibres of the isthmus ; the other passes between the 

 two portions of the anterior pyramids. 



3. The transverse fibres of the valve of Vieussens and those of the white 

 commissure, which have been already noticed. 



The grey substance of the isthmus, which now remains to be mentioned, is far 

 from being so abundant as the white substance, and — as in the spinal cord — it is 

 principally situated deeply in the substance of the organ — at least in the greater 

 number of points. 



If the grey matter of the cord is traced into the medulla oblongata, it will be 

 found that it undergoes modifications in its distribution, with regard to the course 

 and displacement of the columns of fibres. The inferior cornua are divided into 

 two portions, in consequence of the decussation of the infra-lateral columns. Their 

 bases are spread along the middle fissure on the floor of the fourth ventricle, 

 where they form the nuclei of the cranicd motor nerves. The heads, carried 

 backwards and outwards, constitute the motor nuclei of the mixed cranicd nerves. 

 The superior cormui are also divided into two portions on entering the medulla 

 oblongata, by the decussation of the superior columns. The base, which accom- 

 panies the central canal of the spinal cord, is spread on the floor of the fourth 

 ventricle, and forms — to the outer side of the nuclei of the motor nerves — the grey 

 mass known as the sensitive nuclei of the mixed cranicd nerves. The head is 

 abruptly thrown outwards, and constitutes the sensitive nucleus — the origin of 

 the large branch of the trigemini. 



To these grey formations must be added the masses alien to the spinal cord. 

 "We mention : the pyramided nuclei, which occupy the inner border of the 

 pyramids ; the olivary nuclei, that are found in other than Soliped animals ; the 



