LEMNISCUS MEDIALIS 497 



boundary of the upper part of the fourth ventricle. There, it 

 should be located, and thence it should be traced backwards into 

 the hemisphere of the cerebellum, of the same side, to the 

 dentate nucleus, and forwards, medial to the lateral lemniscus 

 and ventral to the inferior colliculus, into the tegmental part 

 of the cerebral peduncle. As the fibres are traced into the mid- 

 brain they will be found to approach the corresponding fibres 

 of the opposite side, with which they decussate, in the lower part 

 of the mid-brain, dorso-medial to the medial lemniscus. After 

 decussating they pass upwards to the red nucleus of the opposite 

 side, where the majority terminate. 



When the dissections described are completed the dissector 

 should make a series of transverse sections through the opposite 

 half of the medulla oblongata and the pons, or, better still, 

 through the whole of another specimen, if it can be obtained. 

 In such a series of sections he will be able to note some of the 

 points now to be mentioned, but the majority of the details noted 

 are well seen only on specially prepared and stained sections. 



Internal Structure of the Medulla Oblongata. When transverse 

 sections are made through the upper part of the medulla oblongata, a faint 

 line, called the median raphe, is seen in the median plane. It divides 

 the medulla oblongata into two exactly similar halves. The raphe is 

 formed by the close intersection of fibres running in opposite directions. 



Each half of the medulla oblongata is composed of (a) strands of white 

 matter ; () grey matter, which is present both in the form of direct con- 

 tinuations into the medulla oblongata of portions of the grey matter of 

 the spinal medulla, and in the form of isolated masses, which are not 

 represented in the spinal medulla ; (c] the formatio reticularis, a substance 

 which is composed of grey matter coarsely broken up by fibres which 

 traverse it in different directions ; and (d) neuroglia. The white matter, as 

 in the spinal medulla, is disposed, for the most part, on the surface and the 

 grey matter in the interior, but in the open part of the medulla oblongata 

 the grey matter comes to the surface on its dorsal aspect, and forms the 

 obex (p. 496). 



When the grey matter of the spinal medulla is traced up into the medulla 

 oblongata, many striking changes in its arrangement become apparent. 

 Owing to the increase in size of the large wedge-shaped gracile and cuneate 

 fasciculi, the posterior columns of grey matter are pressed laterally, so that 

 they soon assume a position at right angles to the median plane. At the 

 same time, the cuneate and gracile nuclear columns of grey matter, which 

 grow out from the basal portion of the posterior column and underlie the 

 strands of the same name, begin to make their appearance. From the 

 deep aspects of those nuclei, fibres, which take origin within them, stream 

 antero-medially through the neck of the posterior grey column to reach the 

 ventral median raphe. And as they pass ventrally they separate the head 

 from the basal part of the posterior grey column. The basal part of the 

 grey column remains close to the central canal, but the head and the 

 substantia gelatinosa remain near the surface, and, towards the upper part 

 of the lower half of the medulla oblongata, the head enlarges and forms a 

 prominence on the surface which has already been noticed as the tuberculum 

 cinereum (p. 481). 



The fibres which spring from the cells of the nucleus gracilis and the 



nucleus cuneatus and break through the neck of the posterior grey column 



are called the internal arcuate fibres. They reach the raphe on the deep 



or dorsal aspect of the pyramidal fasciculus and, in the median plane at the 



VOL. Ill 32 



