THE MID-BRAIN OR MESENCEPHALON 803 



WHITE SUBSTANCE. (1) The origin and course of the superior peduncle have 

 already been described (page 792). 



(2) The medial (posterior) longitudinal fasciculus is continuous below with the 

 proper fasciculi of the anterior and lateral funiculi of the medulla spinalis. In 

 the medulla oblongata and pons it runs close to the middle line, near the floor 

 of the fourth ventricle; in the mid-brain it is situated on the ventral aspect 

 of the cerebral aqueduct, below the nuclei of the oculomotor and trochlear 

 nerves. Its connections are imperfectly known, but it consists largely of ascend- 

 ing and descending intersegmental or association fibers, which connect the 

 nuclei of the hind-brain and mid-brain to each other. Many of the fibers arise 

 in Deiters's nucleus (lateral vestibular nucleus') and divide into ascending and descend- 

 ing branches which send terminals and collaterals to the motor nuclei of the cranial 

 and spinal nerves. Its spinal portion is located in the anterior funiculus and is 

 known as the vestibulospinal fasciculus. Other fibers pass to the median longitudinal 

 bundle from cells in the reticular formation of the medulla, pons and mid-brain 

 and also from certain large cells in the terminal nucleus of the trigeminal nerve. 

 According to Edinger it extends to the so-called nucleus of the posterior longi- 

 tudinal bundle in the hypothalamic region, but this is uncertain and the fibers 

 above the nucleus of the oculomotor are smaller in diameter than the rest of the 

 bundle. According to Held fibers from the posterior commissure can be traced 

 into the posterior longitudinal bundle, and according to the same author many 

 of the descending fibers arise in the superior colliculus, and, after decussating in 

 the middle line, end in the motor nuclei of the pons and medulla oblongata. These 

 fibers from the superior colliculus probably pass into the ventral longitudinal 

 bundle. Fibers are said to pass through the medial longitudinal fasciculus from the 

 nucleus of the abducent nerve into the oculomotor nerve of the opposite side, and 

 through this nerve to the Rectus medialis oculi. Eraser, however, denies the exist- 

 ence of such fibers. Again, fibers are said to be prolonged through this fasciculus 

 from the nucleus of the oculomotor nerve into the facial nerve, and are distributed 

 to the Orbicularis oculi, the Corrugator, and the Frontalis. 1 



The ventral longitudinal bundle consists for the most part of the tectospinal fas- 

 ciculus, and arises from the superior colliculus, the fibers arch ventrally around the 

 central gray matter and cross the midline in the fountain-decussation of Meynert. 

 They then descend in the tegmentum, part of them passing through the red nucleus 

 ventral to the medial longitudinal bundle. In the medulla oblongata and spinal 

 cord its fibers are more or less intermingled with the medial longitudinal bundle 

 and the rubrospinal tract. It descends in the adjoining region of the ventral 

 and lateral funiculi. Collaterals and terminals are given off to the red nucleus and 

 probably other nuclei of the brain stem and to the anterior column of the spinal 

 cord. It is probably concerned in optic reflexes. 



(3) The medial lemniscus or medial fillet (Fig. 713). The fibers of the medial 

 lemniscus take origin in the gracile and cuneate nuclei of the medulla oblongata, 

 and as internal arcuate fibers they cross to the opposite side in the sensory decussa- 

 tion (page 777). They then pass in the interolivary stratum upward through 

 the medulla oblongata, in which they are situated behind the cerebrospinal fibers 

 and between the olives. In the pons and lower part of the mid-brain it occupies 

 the ventral part of the reticular formation and tegmentum close to the raphe, while 

 above it gradually shifts to the dorso-lateral part of ths tegmentum in the angle 

 between the red nucleus and the substantia nigra. In the pons it assumes a flattened 

 ribbon-like appearance, and is placed dorsal to the trapezium. As the lemniscus 

 ascends, it receives additional fibers from the terminal sensory nuclei of the.cranial 





1 A. Bruce and J. H. Harvey Pirrie, "On the Origin of the Facial Nerve," Review of Neurology and Psychiatry t 

 December, 1908, No. 12, vol. vi, produce weighty evidence against the view that the facial nerve derives fibers from 

 the nucleus of the oculomotor nerve. 1 



