MEDULLA OBLOXGATA. 



503 



the crura cerebri, descends into the pons Varolii, and through it is 

 continued into the medulla oblongata, whilst another part joins the 

 cerebellum. Situated on the fibres which extend up from the con- 

 stricted part, are a series of eminences, named, from behind forwards, 

 the corpora quadrigemiua, optic thalami, and corpora striata ; and 

 springing from the front and outer side of the corpora striata are the 

 large convoluted cerebral hemispheres, which expand from this place in 

 all "directions, concealing the eminences named, and occupying the vault 

 of the cranium, the anterior and middle cranial fossae, and the superior 

 fossa? of the occipital bone. The cerebral hemispheres are united to- 

 gether by a principal and smaller commissures ; by means of which 

 there is enclosed a cavity, which is subdivided into various ventricles, 

 viz., the two lateral, the third, and the fifth. 



THE MEDULLA OBLONGATA. 



The medulla oblongata is bounded above by the lower border of the 

 pons Varolii, whilst it is continuous below with the spinal cord, on a 

 level with the upper border of the atlas, at a point which corresponds 

 with the lower extremity of the anterior pyramids, to be presently 

 described. It inclines obliquely downwards and backwards ; its ante- 

 rior surface rests in the basilar groove, whilst its posterior surface is 

 received into the fossa named the vallecula, between the hemispheres of 

 the cerebellum, and there forms the floor of the fourth ventricle. To 

 its sides several large nerves are attached. 



Fig. 354. — View of the Anterior Surface Fig. 354. 



OF THE Pons Varolii and Medulla Ob- 

 longata. 



a, a, anterior pyramids ; &, their decussation ; 

 c, c, oliTary bodies ; d, d, restiform bodies ; c, 

 arciform fibres ; /, fibres described by Solly as 

 passing from the anterior column of the cord to 

 the cerebellum ; ff, anterior column of the 

 spinal cord ; h, lateral column ; p, pons Varolii ; 

 i, its upper filjres ; 5, 5, roots of the fifth pair 

 of nerves. 



The term medulla ol^longata, as employed 

 by AVillis and Vieussens. and by those who 

 dii-ectly followed them, included the crura 

 cerebri and pons Varolii, as well as that 

 part to which by Haller first, and by most 

 subsequent writers, this term has been re- 

 stricted. 



It is of a pyramidal form, having its 

 broad extremity upwards : it is expanded 

 laterally at its upper part : its length 



from the pons to the lower extremity of the pyramids is about an inch 

 and a quarter ; its greatest breadth is nearly an inch ; and its thickness, 

 from before backwards, is about three-quarters of an inch, 



Pissures. — The anterior and posterior median fissures which partially 

 divide the spinal cord are continued up into the medulla oblongata. 

 The anterior fissure terminates immediately below the pons in a recess, 



