THE ISTHMUS. 769 



/ibres, between the anterior border of which and the transverse 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 corpus oUvare} This is isolated from the pyramid by a longitudinal groove, 

 whence emerge, in front, the roots of the sixth cranial pair, and behind, those of 

 the twelfth pair ; 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. 424, 5), which constitutes the floor of the fourth ventricle. 

 This cavity is prolonged forward above the pons Varolii, between the crura 

 cerebelli, and from its forming an angle behind, 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. 425, 1). 



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

 borders, these faces give the profile of the corpora restiformia (Fig. 426, 2), 

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



2. The Pons Varolii (Figs. 425 ; 426, 5). 



TliQpons 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, and which is lodged in the anterior 

 depression of the basilar process. 



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

 bridge, from one side to the other of the cerebellum. It has two faces. The 

 superior face is adherent to the fasciculi of the medulla oblongata, which is 

 prolonged to form the crura cerebri.^ The infeiior face is in every sense 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. It offers for consideration 

 two borders and two extremities. 



The posterior border, sUghtly convex, is separated from the medulla oblongata 

 by a slight groove. 



* This prominence corresponds to the corpus olivare of Man only in its position, as it has 

 not its structure ; for the former has a grey nucleus in its substance, which is divided into two 

 email masses, the superior of whicli is found in Ruminants. 



* In the bulbar portion of the calamus gcriptarius, are seen the diversely coloured prominences 

 which correspond to the origin or course oi' some cranial nerves. These are— viewing them 

 from behind to before, and within to without, commencing at the middle fissure — the internal 

 white alu, corresponding to the nucleus of the hypoglossal nerve; the grey ala, corresponding 

 to the nuclei of the pneumogastric, spinal, and glosso-pharyngeal nerves ; the external white 

 ala, situated above the auditory nucleus; lastly, the eminentia teres, placed between the 

 middle fissure and the crura cerebelli, lodging the bend in the root of the facial nerve. 



The point of the calamus scriptorius is covered by a small white layer extending from one 

 border to the other, named the holt, flie cavity beneath this being designnted the ventricle of 

 Arantius, at the bottom of which is usually seen the opening of the central canal of the spinal cord. 



* The posterior pyramids are formed by the bulbar prolongation of the grey column of the 

 epinal cord ; they pass to the right and left, and disappear towards the posterior crura cerebelli ; 

 they expand near the summit of the calamus scriptorius, and at this point possess very great 

 excitability. 



* Some authorities carry the superior face of the pons to the floor of the fourth ventricle. 



