THE BRAIN AND ITS MEMBRANES. 



741 



transversely, and meet each other on the side of the mid-brain, being separated 

 only by a groove, the sulcus lateralis, which runs from below upward and forward. 

 The dorsal surface is free, but is concealed from view, from above, in the com- 

 plete brain by the overhanging hemispheres. The ventral surface is also free, 

 and also concealed, from below, by portions of the hemispheres, apices of tem- 

 poral lobes, which overlap it. These two surfaces are not parallel, as the ventral 

 surface, besides being convex from side to side, is slightly concave from below 

 upward and forward. The cavity of the mid-brain is the smallest of all the brain 

 ventricles." It is called the aqueduct of Sylvius, and is a mere tube whose 



N. opticu* 



Corp. f* 

 quadr ^-\po is t 



Inf. Hypophysis 

 . oculomotor 



y. access. 



-V. facialis 



'. acusticus 



hypoglossus 



Ant. roots of the 

 Jirst spinal nerves 



FIG. 439. Medulla, pons. and mid-brain seen from the right side, X \. (Gegenbaur.) 



diameter is very small compared to the bulk of the mid-brain in which it lies. It 

 is situated close under the dorsal surf ace in the middle line; hence its direction 

 is upward and forward. It opens below into the fourth ventricle and above into 

 the third ventricle. 



Main Divisions. The mid-brain, as a whole, is divided into two portions (Fig. 

 440), a postero-superior and an antero-inferior, by a lamina of gray matter, the 

 substantia nigra, which is seen to be convex 

 downward and from side to side in sections 

 made both dorso-ventrally and from above 

 downward. Hence this lamina, as a whole, has 

 an antero-posterior curve, with the concavity 

 looking ventrally. Its two edges lie along and 

 in the sul"ii.<* lateralis. 



All of the mid-brain dorsal to and above (pos- 

 tero-superior) the substantia nigra is called the 

 tegmentum. while that portion which is below and 

 anterior (antero-inferior) is known as the crustce. 



Crustse. There are two crustae, which diverge from each other from below 

 upward. The lower end of each is overlapped by the upper margin of the tuber 

 annulare of the pons. Each crusta is a thick, wide, rounded bundle of longitudi- 

 nal white fibres, its outer margin being limited by the sulcus lateralis. Its inner 

 margin is free, and in the interval between it and the opposite crusta is the sub- 

 stantia nigra passing across. Dorsal to each crusta is the substantia nigra, and 

 on the inner margin of each, where the substantia nigra is about to cross over, is 

 a groove, the mesial sulcus, or sulcus oculo-motorius, out of which the roots of the 



FlG " ^- 



Corpus quad- 

 rigem. ant. 

 Aquxducius 



Sylvii 

 Tegmentum 



Subst. nigra 



Crusta 



mid-brain, 



