5H THE MEDULLA OBLONGATA. 



the floor of which is the grooved posterior surface of the medulla oblongata, 

 and which is named the fourth ventricle of the brain. Oil each side of 

 this, the cerebellum is connected with the medulla oblongata and cerebrum, 

 and also receives the fibres of the pons Parolii, which is a commissure 

 passing beneath and between the fibres which extend upwards from the 

 medulla oblongata, so as to unite the two lobes of the cerebellum. 



The cerebrum includes all the remaining and much the largest part of the 

 encephalon. It is united with the parts below by a comparatively narrow 

 and constricted portion or isthmus, part of which, forming the crura cerebri, 

 descends into the pons Varolii, and through it is continued into the me- 

 dulla oblongata, whilst another part joins the cerebellum. Situated on the 

 fibres which extend up from the constricted part, are a series of eminences, 

 named, from behind forwards, the corpora quadrigemina, 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 fossse, and the 

 superior fossse of the occipital bone. The cerebral hemispheres are united 

 together by 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 ollongata 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 anterior 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. 



The term medulla oblongata, as employed by Willis, by Yieussens, and by those 

 who directly followed them, included the crura cerebri and pons Varolii, as well as 

 that part between the pons and the foramen magnum, to which, by Haller first, and 

 by most subsequent writers, this term has been restricted. 



It is of a pyramidal form, having its broad extremity directed 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 back- 

 wards, is about three-quarters of an inch. 



The anterior and posterior mesial 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, the foramen ccecum of 

 Vicq d'Azyr ; the posterior fissure is continued upwards into the floor of the 

 fourth ventricle, where it opens and expands in a superficial furrow, and is 

 gradually lost. 



In other respects an entirely different arrangement of the parts prevails 

 from that in the cord. The surface of each half of the medulla presents 

 four eminences or columns, which are met with in the following order, 

 from before backwards, viz. : the anterior pyramids, the olivary bodies, the 

 restiform bodies, and the posterior pyramids. 



