PAPIO C^'NOCEPHALUS 305 



sections. It has been the purpose rather to select those structures which are 

 more plastic in the evohitional sense and therefore show a greater range of 

 morphological variabihty than the archaic, rigidly fixed portions of the 

 brain stem. This makes necessary the omission of much description which 

 might be desired by students requiring a general histological review of the 

 various levels of the axis, and while such omission is to be regretted, the 

 inclusion of more histological detail would so encumber the text as to make it 

 needlessly burdensome. 



LEVEL OF THE PYRAMIDAL DECUSSATION (FIG. 1 44) 



At this level the most conspicuous features of the oblongata are the 

 crossing fibers of the pyramidal system (Pyx) which have brought about 

 the separation of the ventral gray column (Ven) from the central 

 gray matter (Cen). The width of the pyramidal fascicles forming the 

 decussation is relatively broad, giving the impression of a pyramidal 

 tract with considerable capacity for voluntary control over the animal's 

 musculature. 



In the dorsal fields another feature of importance appears in the caudal 

 extremity of the nucleus of Goll (NG). This nucleus lies immediately 

 lateral to the dorsomedian septum and is surrounded by a dense bundle 

 of fibers constituting the column of Coll (CG). This extensive field of 

 myelinized fibers arises from levels in the spinal cord particularly represent- 

 ing the dermatomes in the skin and the proprioceptors in the muscles, joints 

 and bones of the lower extremities and tail. The column of Goll (CG) is 

 separated from the next adjacent bundle of fibers, the column of Burdach 

 (CB), by a fairly well-dehned dorsal paramedian sulcus. The column of 

 Goll is considerably less in its dimensions than the column of Burdach, 

 warranting the inference that the inllux of stimuli conducted by way of Bur- 

 dach's fasciculus is of greater volume than that passing in by way of the 



