THE MID-BRAIN 901 



(4) the decussating superior cerebellar peduncles, (5) the decussating rubrospinal 

 tracts, and (6) the central tegmental tracts. 



The medial longitudinal bundle lies on each side of the median plane, just ventrad 

 of the central aqueduct gray in the mid-brain and continuous throughout the brain 

 stem in its formatio reticularis. It is the continuation and the equivalent, but 

 in a more differentiated form, of the ventral basis bundle of the spinal cord. 

 It is formed by association neurones and acts as an associating agent with regard 

 to many cranial and spinal nerve centres for the performance of certain definite 

 functions. Its neurones receive impulses from afferent elements and transmit 

 them to motor or efferent elements. It particularly brings into relation the sensor 

 cranial nerve nuclei and the corpora quadrigemina with the motor nerves of 

 the eye (III, IV, and VI), of the face (VII), and of the trunk. A special nucleus 

 for the bundle is described as being situated in the gray floor of the third ventricle, 

 at its junction with the aqueduct. The axones from the cells of this nucleus cross 

 to the opposite side through the posterior commissure (Fig. 662). 



The lateral lemniscus, we have learned, is a continuation of the auditory path 

 in its course to the cerebral cortex. Its formation is described on page 883. In 

 the mid-brain the fibres of the lateral lemniscus course through the lateral part 

 of the tegmentum, near the surface, and most of them end in the gray nucleus 

 of the inferior corpora quadrigemina and in the internal geniculate body. A few 

 fibres are carried into the superior quadrigeminal body. 



The medial lemniscus, or principal conduction path for sensor impulses from 

 the trunk and extremities, and already discussed in the preceding (p. 878), ascends 

 in the tegmentum of the mid-brain in the contact zone with the crusta. In its 

 ascent it is deflected slightly dorsolaterad by the red nucleus. The lateral border 

 of the ribbon-like bundle is in contact with the lateral lemniscus, and forms an 

 angle with it, as seen on trans-section (Figs. 660 and 661). 



Many of the fibres of the medial lemniscus terminate in the superior quadri- 

 geminal body; the remainder proceed to the thalamus. 



The superior peduncles or prepeduncles of the cerebellum sink into the mid-brain 

 tegmentum in a cephaloventral direction, the two superior peduncles converging 

 and their fibres undergoing a complete decussation (W ernekinck' s commissure) 

 subjacent to the inferior quadrigeminal body. The crossed fibres end, for the 

 most part, in the red nucleus of each side; others circumvent the nucleus, forming 

 a white capsule for it which is thicker on its ental surface, and proceed to the thal- 

 amus. 



The tractus rubrospinalis (Monakow's) is composed of axones arising in the 

 red nucleus, decussating with those of the opposite tract, and descending in 

 the tegmentum to the lateral intermedial fasciculus of the cord, to terminate 

 in relatio'n with ventral-horn cells. 



The central tegmental tract (olivary fasciculus} probably arises in the inferior 

 olivary nucleus and ascends in the tegmentum. In the pars dorsalis pontis 

 it is best seen in trans-sections as a compact longitudinal bundle along the dorsi- 

 mesal aspect of the superior olive. Cephalad it is said to end in the lenticular 

 nucleus. 



Fountain Decussation. 1 A dense decussation may be found in the space between 

 the two red nuclei. The fibres composing the decussating bundles arise from 

 cells in the superior corpora quadrigemina and central aqueduct gray. After 

 having crossed the middle line they descend as the tectospinal tract, join the medial 

 longitudinal fasciculus, and give off collaterals to, or terminate in the nuclei of, 

 the eye muscle nerves, as well as to spinal centres for movements of the head and 

 neck. 



1 Decusaatio tontinahs, so called because of the resemblance of the scattering strands to the jets of a fountain. 



