I.IGAMENTUM DENTICULATUM FORMED BY THE PIA MATER. 567 



of the spinal marrow of the elephant, a, Is the dura mater; 

 the arachnoid membrane being removed, c, The medulla 

 spinalis, d, The pia mater, closely embracing the medulla, 

 and dipping in, in the form of a fold, to the bottom of the ante- 

 rior median fissure e, and the posterior y*. g, Doubling of the 

 pia mater, forming the ligamentum denticulatum. i, i, The 

 tooth-like processes of the ligament, by which it is inserted into 

 the inner face of the dura mater ; the ganglion k, formed on 

 the posterior root of the nerve, as well as the anterior root, is 

 closely embraced by a process from the dura mater b, forming 

 its neurilema. In this manner the medulla is always effectually 

 maintained in the middle of the lateral diameter of the spinal 

 canal. 



Fig. 223. 



Of the Medulla Oblongata. 



— ^The medulla oblongata, (see fig. 224, page 577,) is obviously 

 a prolongation of the spinal marrow as its name imports, and is 

 the medium of communication, between all the parts of the 

 body which receive nerves from the spinal cord and all the por- 

 tions of the cerebrum and cerebellum above. Hence it has 

 been called the central point of the nervous system, and as it 

 gives origin to nerves, (pneumogastric, etc.,) directly connected 

 with the performance of functions the most immediately neces- 

 sary to life, it is deserving on the part of the anatomist of the 

 most patient study. It occupies the groove in the basilar pro- 

 cess of the occipital bone, and is covered on its posterior surface 

 by the cerebellum. 

 — It is about fourteen lines long, nine lines wide at its largest 



