FIBRES OF OBLONG MEDULLA. 



185 



column of the cord on the opposite side, which remains below the cross 

 fibres (lor the cord has been cut through near these), may be forcibly turned 

 outwards. 



The anterior pyramid (fig. 56, a) receives fibres inferiorly from the 

 anterior column of the cord of its own side, and from the opposite half of 

 the cord. The inner set of fibres, 6, deep at their origin, become super- 

 ficial, and are directed upwards, close to the median fissure, joining the 

 fibres continued from the anterior column of the cord ; and as the inner 

 fibres of each pyramid are derived from the opposite side, they cross each 

 other in the median line forming thus the decussation of the medulla ob- 

 longata. 



The fibres of the pyramid are white and longitudinal, and are collected 

 into a bundle of a prismatic form. Superiorly they enter the pons (fig. 

 56), to reach the cerebrum. 



Dissection. For the purpose of seeing the arrangement of the fibres of 

 the lateral tract, the anterior pyramid is to be cut across on the right side, 

 between its decussation and the olivary body (fig. 57), and to be raised 

 towards the pons. 



The lateral tract of the medulla is prolonged inferiorly into the portion 

 of the spinal cord between the anterior and posterior roots of the nerves. 

 The continuation of the tract (fig. 57) ascends beneath the olivary body, 

 and leaving the surface of the medulla enters the pons. 



The olivary body, and its filet (fig. 57, c). The olivary mass consists 

 of three parts, viz., a gray incasing layer, a nucleus, and a band prolonged 

 from it the fillet. 



Fig. 57. 



a. Anterior pyramid fibres, cut 



through, and raised as far 

 as the optic thalamus. 



b. Lateral tract of the medulla 



oblongata. 



c. Olivary body, and fillet of the 



olivary body issuing at the 

 upper end. 



d. Fibres of the lateral tract in 



the pons and crus cerebri. 



e. Superficial, and 



f. Depp transverse fibres of the 



pons, cut through, and turn- 

 ed outwards. 



g. Locus niger in the crus cere- 



bri, between the fibres of the 

 lateral tract and pyramid. 



FIBRES OF THE LATERAL TRACT AND OF THE OLIVARY BODY. 



When the olivary body has been sliced obliquely, a very thin wavy 

 layer of gray substance surrounding a nucleus of whitish matter will be 

 apparent: this is the corpus dentatum (fig. 58,/). It forms a thin cap- 

 sule or bag, having a zigzag outline in a section, with the dilated part 



