240 THE ANATOMY OF THE HORSE. 



column. It enters the cerebellum, of which it foiTUS the posterior peduncle. 

 The superior pyramid derives its fibres from the innermost part of the 

 superior column of the cord on the same side, and its fibres are continued 

 through the pons to the cerebrum. 



The medulla oblongata shows the superficial origin of the last seven 

 cranial nerves. 



THE PONS VAROLII (PLATE 33). 



The pons Varolii rests on the basilar process, in front of the medulla 

 oblongata. In front of it the crura cerebri appear. Its inferior 

 face is convex in both directions, and has a faint median fuiTow. 

 The superior face forms the anterior part of the floor of the 4th 

 ventricle. Its extremities are curved upwards to enter the cere- 

 bellum, of which they form the middle peduncles. The pons consists 

 of white and grey nerve matter. The nerve fibres of the white 

 matter are airanged in two sets — a transverse and a longitudinal. 

 The transverse fibres consist of the surface fibres of the pons, and 

 of deeper fibres separated from these by the longitudinal set. It is 

 these transverse fibres that curve upwards at either extremity of 

 the pons to enter the cerebellum as its middle peduncle, and they 

 accordingly play the part of a commissiire to the right and left 

 halves of the cerebellum. The longitudinal fibres are the forward 

 continuation of the longitudinal fibres of the medulla oblongata, 

 minus the restiform bodies. In front of the pons these longitudinal 

 fibres are continued as the crura cerebri. The grey matter of the 

 pons occurs Avithiu its substance, and at the floor of the 4th ventricle. 

 To a group of pigmented nerve cells in the latter position, the term 

 locus cceruleus is applied. 



The pons shows the superficial origin of the 5th cranial nerve, by two 

 distinct roots springing from its lateral aspect. 



THE CEREBELLUM (PLATES 34 AND 35). 



The cerebellum is superposed to the medulla and pons, and 

 lies under the supra-occipital division of the occipital bone. The 

 tentorium cerebelli arches downwards in front of it, and isolates 

 it fi-om the posterior extremities of the cerebral hemispheres. It is 

 traversed in the antero-posterior direction by two shallow grooves, 

 which divide it into a middle and two lateral lobes. The middle 

 lobe is the smallest, and is known as the vermiform lobe. When 

 followed forwards, the vermiform lobe is seen to be reflected round 

 the anterior aspect of the cerebellum to gain its lower surface at 

 the roof of the 4th ventricle ; and it terminates by a blunt end 

 about the middle of this surface. When followed posteriorly, the 

 vermiform lobe behaves in the same way, terminating at the roof of 

 the 4th ventricle by a blimt end opposed to the first. These reflected 



