THE ISTHMUS. 677 



band, and particularly in pieces that have been hardened by alcohol or 

 acidulated water, is seen a slight oblong prominence which corresponds to 

 what in Man is designated the olive ^ (^corpus olivare) ; it is isolated from 

 the pyramid by a longitudinal groove, whence emerge, in front, the roots 

 of the sixth cranial pair, behind, those of the twelfth ; outwardly, it is 

 limited and separated from the restiform body by the origin of the majority 

 of the roots belonging to the glosso-pharyngeal and pneumogastric nerves. 



Superior face. — Covered by the cerebellum, it is channeled in its middle 

 by an excavation (Fig. 323, 5), which constitutes the floor of the fourth 

 ventricle. This cavity is prolonged forward above the pons Varolii, between 

 the cerebellar peduncles, and from its forming behind an angle resembling 

 the point of a pen, it has been named the calamus scriptorius. 



Two thick cords, prolongations of the superior fasciculi of the medulla 

 spinalis, border the calamus scriptorius on each side ; these are designated 

 the corpora restiformia. Lying together at their posterior extremities, they 

 separate anteriorly, so as to represent the branches of a V (Fig. 323, 1). 



Lateral faces. — Much narrower than the other two, and showing two 

 thick borders, these faces give the profile of the corpora restiformia (Fig. 

 324, 2), corpora pyramidalia (4), and the fasciculus between these two. 



2. The Pons Varolii.- (Figs. 322, 14 ; 324, 5.) 



The pons Varolii, also named the tuber annulare or mesocephalon, is that 

 part of the brain which stands out prominently across the isthmus, between 

 the medulla oblongata and the crura cerebri, andwhich is lodged in the 

 anterior depression of the basilar process. .—• ir^-- 4|^ 



It is a semicircular band of white transverse fibres thrown across, like # 

 bridge, from one side to the other of the cerebellum. 'In every sense it is 

 convex, wider in its middle than in its lateral portions, and crossed from 

 behind' to before by a shallow median groove for the basilar artery. On its 

 free surface, whose principal features we have just described, it offers for 

 consideration two borders and two extremities. 



The posterior border, slightly convex, is separated from the medulla 

 oblongata by a faint groove. _ ■■,-,■,■, i 



The anterior border, also convex, but indented m its middle, largely over- 

 hangs the crura cerebri, which are limited on this side by a well-marked 



^™The extremities are bent upwards to enter the substance of the cerebellum, 

 in the form of two thick cords, which constitute the middle crura cerebelli 

 (Fig. 324, 6). They exhibit the apparent origin of the trifacial nerves. 

 The pons Varolii does not exist in birds. 



3. The Pedunculi or Crura Cerebri. (Figs. 322, 11 ; 324, 7.) 



These are two very large white fasciculi, visible at the inferior surface 



and sides of the isthmus, covered superiorly by the corpora quadrigemma 



and thalami optici, and continuous, above the pons Varolii, with the fibres of 



the medulla oblongata; while their anterior extremities enter the cerebral 



^'"rhtrpeduncles (or crura) are separated from each other by a middle 

 • This prominence corresponds to the c<yrpus olivare of Man only in its position, for 

 -it' has not its structure. 

 46' 



