720 THE yERVOrS SYSTEM 



hemisphere of the cerebellum. Mesially, it is marked by a sagittal groove or 

 furrow, which usually lodges the basilar artery, Init is in no way caused by the 

 contact with the vessel. On each side of the groove a prominence is formed, where 

 the sagittally-directed tibres of the pyramidal bodies pass brainwards, under cover 

 of the transverse tilires of the pons. 



The superior border is convex forwards. Its fibres pass directly into the 

 upper border of the middle i)eduncle of the cerebellum. 



The inferior border is marked off by a distinct transverse groove from the 

 medulla. It is formed in its inner part by the lowest fil^res of the pons, but these 

 are crossed near the junction of pons and middle peduncle by the oblique fasciculus. 

 The oblique fasciculus is formed by the middle transverse fibres of the pons, 

 which pass downwards and backwards, internal to the exit of the trigeminal nerve, 

 on each side, and cross the inferior fibres on their superficial aspect. 



Coronal sections through the pons (fig. 429) shoAV that it comprises two very 

 distinct regions — an anterior or ventral region and a dorsal region, which is called 

 tegmental, as most of its constituents are continued into the tegmentum of the 

 mesencephalon. The ventral region consists of transverse and longitudinal fibres, 

 interspersed with small aggregations of grey matter. The transverse fibres are: — 

 (a) Fibres which pass from the middle peduncle to the opposite side, where they 

 turn upwards to the brain or downwards to the cord; (6) fibres which emerge 

 from the cerebellum by the middle peduncles, and terminate in connection with 

 the nuclei of the pons on the same side. The longitudinal fibres are the upward 

 continuation of the pyramids. They pass upwards in flattened bundles, separated 

 from one another by some of the transverse fibres of the pons. 



In the upper or tegmental region the chief things to be observed are: the 

 tract of the fillet (which lies next to the ventral region), the formatio reticularis, 

 the posterior longitudinal bundle, the substantia ferruginea, and the funiculus teres. 

 The superior olivary nucleus, the nuclei of the sixth and seventh cranial nerves, 

 the motor nucleus of the trigeminal, with parts of the ascending and descending 

 roots of the last-named nerve and portions of the nucleus of the auditor}' nerve, are 

 embedded in this region of the pons (figs. 429 and 430). 



The dorsal and ventral parts of the pons are separated by a transverse layer of 

 fibres called the trapezium or corpus trapezoides (fig. 430), because in some 

 mammals it appears on the ventral surface of the pons in a quadrilateral area 

 between the bundles of the pyramidal fibres of the two sides. It consists of fibres 

 crossing from side to side through the raphe many of which, derived from the 

 accessory auditory nucleus, pass through the trapezium to the lateral fillet of the 

 opposite side and by it are conveyed to the lower quadrigeminate body. 



Some of the structures mentioned in the preceding paragraph are visible to the unaided eye 

 in ordinary dissecting-room specimens. The nuclei of the cranial nerves will not be further 

 described here, as a full account of them will be given in the section on the cranial nerves. 



The Metexcephalox 



The MEDULLA OBLONGATA, or bulb, is the portion of the cerebro-spinal 

 axis which extends from the inferior border of the pons to the decussation of the 

 pyramids. It presents an anterior, a posterior, and two lateral surfaces. The 

 anterior surface rests upon the basilar groove of the occipital bone, and extends 

 downwards as far as the point Avhere the odontoid process is crossed by the trans- 

 verse ligament of the atlas (Testut). The bulb has the form of a truncated cone, 

 the base of which is continued into the pons above, while the truncated apex is 

 prolonged into the spinal cord below. The posterior surface is received into the 

 vallecula, between the cerebellar hemispheres, and forms, by its upper part, as 

 already descril)ed, the lower part of the floor of the fourth ventricle. The 

 ninth to the twelfth nerves (with the exception of the spinal part of the spinal 

 accessory) take their superficial origin from its sides. The sixth, seventh, and 

 eighth nerves appear in the transverse groove between the medulla and the pons in 

 numerical order from within outwards. 



The ventricular part of the medulla has been already described; but on the 



