THE CENTRAL NERVOUS SYSTEM. 



77 



lie dorsally to the substantia nigra form the upward continuation of 

 the fillet, which is here bent at a right angle so that it is divisible into 

 a medial and a lateral portion. The medial part is continued upwards 

 to the thalamus, but the larger part of the lateral portion ends in the 

 inferior colliculus of the corpora quadrigemina. The other tracts of 

 fibres found in the tegmentum are the posterior longitudinal bundle 

 (medial longitudinal fasciculus) and the superior peduncle. The 

 tegmentum also contains a certain amount of grey matter, the greater 

 part of which forms a well-defined group of nerve cells, known as the 



Inferior colliculi. 



Cerebral aqueduct. 



Nucleus of oculomotor 

 nerve. 



Lateral fillet. 



Posterior longi- 

 tudinal bundle. 



Medial fillet. 



Raphe. 



FIG. 22. Transverse section of mid-brain at level of inferior colliculi. 

 (Gray's Anatomy. ) . 



red nucleus, lying near the middle line. The red nucleus gives origin 

 to the fibres of the rubro-spinal tract, and receives most of the fibres 

 of the superior peduncle. 



The corpora quadrigemina are composed of grey matter, but the 

 superior colliculi are covered by a layer of white fibres derived from 

 the optic tract. Most of the fibres of the lateral fillet end in the 

 inferior colliculus, the majority in that of the same side, some, however, 

 crossing to the opposite side. The inferior colliculus is also connected 

 by a bundle of fibres, the inferior brachium, with the internal (medial) 

 geniculate body. Other fibres connect it, by way of the tegmentum, 

 with the thalamus and the temporal lobe of the cerebral hemisphere. 

 These various connections form part of the auditory tract. The lateral 

 fillet conveys impulses from the nuclei of the cochle&r nerve. These 



