CHAPTER XI 



Lamina quadrigemina 



Cerebral aqueduct / 



Central gray stratum-^., .(^ \J 



Tegmentum 



THE INTERNAL STRUCTURE OF THE MESENCEPHALON 



A DIAGRAM of a transverse section through the rostral part of the mesen- 

 cephalon will make clear the relation of the various parts of the midbrain to 

 each other (Fig. 113). The cerebral aqueduct is surrounded by a thick lamina 

 of gray matter, the central gray stratum (stratum griseum centrale). Dorsal to 

 this lies the lamina quadrigemina, a plate of mingled gray and white matter 

 which bears four rounded elevations, the corpora quadrigemina. The ventral 



part of the midbrain is formed by 

 the cerebral peduncles, each of which 

 is separated into two parts by a 

 lamina of pigmented gray substance, 

 known as the substantia nigra. 

 Dorsal to this the peduncle consists 

 of reticular formation continuous 

 with that of the pons and known as 

 the tegmentum. Ventral to the sub- 

 stantia nigra is a thick plate of longitu- 

 dinal fibers, called the basis pedunculi, 

 composed of fibers which are continuous with the longitudinal fasciculi of the 

 pons. 



The Tegmentum. The dorsal portion of the pons is directly continuous 

 with the tegmentum of the mesencephalon. Both are composed of reticular 

 formation, consisting of interlacing longitudinal and transverse fibers grouped 

 in fine bundles and separated by minute masses of gray substance, in which are 

 embedded important nuclei and fiber tracts. In the caudal part of the mid- 

 brain and the rostral part of the pons are four cellular masses the locations of 

 which are indicated in Fig. 112. They are the dorsal nucleus of the raphe, the 

 superior central nucleus, the ventral tegmental nucleus, and the dorsal tegmental 

 nucleus. The latter is a collection of small cells in the central gray substance, sep- 

 arated from the ventral tegmental nucleus by the medial longitudinal bundle. 

 Both the ventral and dorsal tegmental nuclei receive fibers from the mammillary 

 body (tractus mamillotegmentalis) , and within the dorsal one there also ter- 



Basis pedunculi'" 



Substantia nigra--'' 

 Fig. 113. Diagrammatic cross-section through 

 the human mesencephalon. 



