708 



THE NERVOUS SYSTEM. 



and of the medulla is described as one cavity under the name of the fourth 

 ventricle. These spaces all communicate with one another (Fig. 413). Thus the 

 fourth ventricle opens above into the aqueduct of Sylvius, which in its turn leads 

 into the back part of the third ventricle, and this, from its front portion laterally, 

 communicates with each lateral ventricle by means of the corresponding foramen 

 of Monro. It is thus seen that this foramen was originally the simple orifice 

 formed by the protrusion of the hemisphere vesicle from the side of the prosen- 

 cephalon (Fig. 414). 



THE MEDULLA OBLONGATA (Figs. 415 and 416). 

 General Description. 



The medulla oblongata, or spinal bulb, is the first division of the brain, pro- 

 ceeding from below upward. It has two extremities, superior and inferior, and 

 four surfaces, dorsal, ventral, and two lateral. The inferior extremity is directly 

 connected with the spinal cord ; the upper has a similarly direct connection with 

 the pons Varolii (Fig. 415). The surfaces in the upper half of the medulla are 

 distinct from each other ; in the lower half each runs into the other by insensible 



Brachium' 

 corp. quad- 

 rig, post. 



FrsenuLum 



Crusta N. trochle- 

 (mid-brain) aris 

 -Jf. trig. Lingula 



Basilar 

 sulcus 



Olive 

 Pyramid 



Decussation of 

 the pyramids 



Funiculus 

 teres 



Epiphysis 



Restiform 

 body 

 Ligala 



Obex 



Strife aci 



Ala cinerea 

 Clava 



. Fun 



S ((o 





the left of the li 



Funiculut ffracilis (to 



the left of the line) 



Post. med. fissure 



FIG. 415. Ventral surfaces of medulla, pons, and 

 mid-brain. (Gegenbaur.) 



FIG. 416. Dorsal surfaces of medulla, pons, and 

 mid-brain, c. q.a. and c.q. p., corpora quad. aut. and 



Eost. adpont.= cut surface of middle ped. of cerebel- 

 im. act med. = cut surface of inf. ped. of cerebel- 

 lum, ad cer. = cut surface of sup. ped. of cerebellum. 

 (Gegenbaur.) 



gradations. Hence the outline of a cross-section of the upper half would show 

 each of these surfaces distinctly, while a similar outline of the lower half would 

 be almost that of a circle. 



The lateral diameter of the medulla increases from below upward, that of the 

 lower end being about equal to that of the cord, while that of the upper is but 

 little less than that of the pons. The dorso-ventral diameter also increases 

 slightly from below upward, but is always less, at any given level, than the 

 corresponding lateral diameter. Hence the medulla is somewhat flattened dorso- 

 ventrally and expands laterally as it ascends. It is directed obliquely from below 

 upward and forward, and its lower end, which joins the cord, is on a level with 

 the lower margin of the foramen magnum. Its ventral surface rests on the 

 basilar groove of the occipital bone, while its dorsal surface lies under the space 

 which separates the two hemispheres of the cerebellum. Ventrally its upper end 



