590 HANDBOOK OF PHYSIOLOGY. 



of the opposite side ; the rest of the pyramid being made up of the fibres 

 from the anterior column of the cord known as the direct or uncrossed 

 pyramidal fibres. These two pyramidal strands of fibres are those which 

 degenerate on lesions of certain parts of the cerebrum which are known 

 as the motor areas of the cortex. They can therefore be traced downward 

 on such lesions as tracts of degeneration. They are the fibres of commu- 

 nication between the cerebral cortex and the different segments of the 

 spinal cord. The anterior pyramids of the bulb are marked out by the 

 exit from that part of the nervous axis to the outside of them, of a 

 nerve, the 12th or hypoglossal. More laterally than this nerve, there 

 soon becomes very prominent on either side a rounded elevation or col- 

 umn which is known as the olivary body. It is not seen at the begin- 

 ning of the bulb at its junction with the cord, but begins at a lower 

 level than the opening of the fourth ventricle. On the further side of 

 the olivary body is seen the line of origin of fibres of the llth, 10th, and 

 9th nerves, and from this to the posterior fissure is the posterior pyramid. 



The whole of that part of the medulla which is situated laterally 

 between the olivary body and the posterior fissure is known as the resti- 

 form body; it is continued forward on either side as the inferior peduncle 

 of the cerebellum. 



The changes which are noticed by the study of series of sections of 

 the bulb from below upward may be summarized thus: In the dorsal or 

 posterior region, the posterior cornua are pushed more to each side, and 

 the substance of Rolando is increased and becomes rounded, reaching 

 almost to the surface of the bulb on each side, a small tract of longitu- 

 dinal fibres of the ascending root of the 5th nerve only intervening. 

 There is a great increase of the reticular formation around the central 

 canal, and the lateral approaches the anterior cornu. Then at the ven- 

 tral or anterior aspect the decussation of the lateral fibres begins. By 

 this crossing over of the fibres, the tip of the gray anterior cornu is cut 

 off from the rest of the gray matter. The central canal is pushed further 

 toward the posterior surface, first of all by the decussation of the anterior 

 pyramids just mentioned, and later on, i.e., above, by another decussa- 

 tion of fibres more dorsal. These fibres of the second decussation as 

 they cross form a median raphe and also help to break up the remaining 

 gray matter into what is called a reticular formation. There has been 

 some little doubt as to the origin of these descussating fibres, but the 

 best authorities now consider them to be, at any rate in part, the fibres 

 from the nuclei of the fasciculus gracilis and fasciculus cuneatus of 

 either side, and look upon them as a sensory decussation. At the pos- 

 terior part soon there appear in the columns of white matter of the 

 fasciculus gracilis and fasciculus cuneatus new masses of gray matter. 

 The lateral fiorn approaches the anterior ; but soon the latter is pushed 



