THE MEDULLA OBLONGATA. 731 



SECTION III. 



MEDULLA OBLONGATA AND PONS VAROLIT. 



The medulla oblongata and the pons are interposed between the 

 il cord and the brain proper. 



Dissection, On a single brain the student may learn nearly all Dissection. 

 tin- anatomy of the medulla and pons ; but if he has a second brain 

 he should cut through the crus cerebri above the cerebellum imme- 

 diately behind the posterior corpus quadrigeminum and then carefully 

 take away the cerebellum from the pons and medulla by cutting 

 through the attachments close to the cerebellum, opening out thereby 

 (fig. 262, p. 733), the fourth ventricle and exposing the upper surfaces 

 of the pons and medulla. 



Position. The brain is to remain in the same position as for the Position, 

 examination of the vessels and nerves. 



The MEDULLA OBLONGATA or BULB is the expanded upper end of Medulla 

 the spinal cord which is contained in the cranium. Its limits are 

 the lower border of the pons in one direction, and the lower margin 

 of the foramen magnum in the other. It is somewhat conical in 

 form, and measures nearly an inch in length, half an inch in form and 

 breadth below, and about an inch at its widest part above. 



Its base joins the pons, the transverse fibres of the latter marking Base, 

 its limit ; and its apex is blended with the spinal cord. The Apex, 

 anterior surface (fig. 261) is irregularly convex, and is in contact with Surfaces, 

 the hollowed basilar process of the occipital bone. The opposite 

 surface (fig. 262) is convex below, and somewhat excavated above, 

 where it forms the floor of the fourth ventricle ; it rests in the 

 groove between the hemispheres of the cerebellum, and on this 

 posterior or upper aspect there are not any cross fibres of the pons, as 

 in front, to mark the limit of the bulb. 



The medulla oblongata is divided into halves by a median fissure Median 

 in front and behind, in a line with those of the cord. The anterior anterior 

 median fissure is interrupted at the lower end of the bulb by some 

 bundles of fibres which cross obliquely from one side to the other, 

 and constitute the decussation of the pyramids ; above, it ends at 

 the lower border of the pons in a dilatation (foramen caecum). The 

 posterior median fissure is continued upwards from the cord through and 

 the lower half of the medulla oblongata, and then terminates by the P sterior< 

 separation of its lateral boundaries to form the sides of the fourth 

 ventricle. 



On each half of the medulla oblongata are elongated eminences, Surface con- 

 separated by slight grooves, and continuous with the columns of the partly con- 

 spinal cord ; but they receive different names in this part of their JJJJ 

 extent, and some fresh bodies are added. Thus, the part continuing new.' 

 the anterior column of the cord, by the side of the anterior median 

 fissure, is known as the anterior pyramid (fig. 261, *). The pro- 

 longation of the lateral column takes the name of lateral tract ( 2 ), the 



