BASE OF BEAIN, OPTIC NERVE, AND COEPOHA QUADEIGEMINA. 383 



to. be sure, are essentially thinner, split itp exactly as they do in the osmatic 

 vertebrates, and pass away as white strands — earlier named olfactory roots — 

 over the gray substance. A lateral tract, often divided into two, may com- 

 monly be separated from a mesial. The former gradually turns inward 

 near to the gyrus hippocampi. At times a thin, light-colored cross-band 

 may be seen to pass over the substantia perforata anterior from without 

 inward and upward. It is nothing other than the atrophied remnant of 

 the olfactory bundle to the eornu Ammonis, which is to be seen in so 

 high perfection in this situation in the osmatic vertebrates. Before leaving 

 the consideration of the base of the brain, turn once more to Pig. 144, be- 

 cause in the osmatic brain there shown so many relations appear clearer; 

 and the structures are better developed. The olfactory apparatus, especially, 

 will then be clearer to you. 



On the mesial edge of the substantia perforata anterior the fibers of the 

 corpus callosum dip down as far as the base of the brain. The elevation they 

 here produce on the inner, mesial cortex of the hemisphere, extending nearly 

 to the base of the brain, is designated as the gyrus subcallosus. Between 

 the two gyri subcallosi there lies a gray plate, which may be traced 

 upward as far as the genu of the corpus callosum. It is the lamina ter- 

 minalis. In it we must recognize a remnant of the embryonic closing plate 

 — that wall which once closed in the primary forebrain, the same wall from 

 which the hemispheres, now of such enormous size, grew and arched over 

 the other parts of the brain. Now, this is only a small gray area of little 

 importance, which lies at the most anterior point of the base of the brain 

 (Fig. 133). 



If an affection involves only the base of the brain in front of the pons, the 

 symptoms that are produced by irritation or paralysis of the nerves lying there will 

 be, by far, the most important for diagnosis. Moreover, disturbances of motility and 

 sensibility may also appear in the extremities if the crura cerebri are involved with 

 the rest. A careful analysis of the symptoms with the aid of an illustration of the 

 base of the brain often leads to a, surprisingly accurate diagnosis of the location of 

 the lesion. 



The optic tracts curve around the crura cerebri and disappear farther 

 ■dorsally in the region of the corpora quadrigemina, particularly in a small 

 ganglion lying adjacent to this laterally, the corpus geniculatum laterale. 

 If you now wish to become better acquainted with the actual termination 

 of the optic nerves, you must first of all give your attention to those parts 

 of the midbrain just mentioned. 



The following illustration shows the corpora quadrigemina when seen 

 from above. They lie upon the peduncles, somewhat crowded in between 

 the thalami. Behind them, on each side, a large fiber-tract comes from be- 

 neath them and sinks into the cerebellum. It is the anterior cerebellar 



