13 CENTRAL NERVOUS SYSTEM. 



THE MEDULLA OBLONGATA. 



The transition from the spinal cord to the medulla oblongata is veiy 

 gradual. Plate XLV. represents the first change which is noticed in the 

 arrangement of the central parts. The central canal — in saurians — be- 

 gins to occupy a lower plane in cross sections, and seeins to have earned 

 with it the inferior cominissure ; and a centimeter behind the fourth ven- 

 tricle — alligator 3 feet long — the canal remains at about the same level, 

 but soon is found in a higher position, and opens into the ventricle as 

 shown in Plate LIL 



Here, in the alligator, the raphe first appears, and continues in the 

 series as far foi-ward as the part which, coiTesponds with the pars com- 

 missuralis of the frog, — Reissner and Stieda — and with the pons Varolii 

 of man. Imbedded in the meshes of the raphe, nene cells begin to ap- 

 pear near the lower border of the medulla. These cells extend forward 

 increasing gradually in size, until a plane is reached just behind the au- 

 ditor}- ners'es, where both the cells and their nuclei have dimensions 

 greater than those of any other cells of the nenous system. 



The larger cells are situated in a horizontal plane higher than that 

 occupied by the smaller cells, and were first described b}' me, a few 

 years ago, as being, so far as their large size and position in the raphe 

 are concerned, characteristic of the alligator. See Plates XLVIIL & 

 Cin. In the alligator, these cells are never found in planes anterior to 

 the auditor}' nerxes. Stieda* found a similar group of cells, in the land 

 turtle of Europe, and named it : "nucleus basilaris." The cells are not 

 * "Ueber den Bau des Centralen NeiTcnsystems." Schildkrote. Loc. cit. p. 51. 



