13 CENTRAL NERVOUS SYSTEM. 



THE MEDULLA OBLONGATA. 



THE transition from the spinal cord to the medulla oblongata is very- 

 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 seems to have carried 

 with it the inferior commissure ; 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 LI I. 



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

 series as far forward as the part which, corresponds 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, nerve 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- 

 ditory nerves, where both the cells and their nuclei have dimensions 

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



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

 occupied by the smaller cells, and were first described by 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 XLVIII. & 

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

 the auditory nerves. Stieda* found a similar group of cells, in the land 

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

 * "Ueber den Ban des Centralen Nervensystems." Schildkrote. Loc. cit. p. 51. 



