56 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



corpus callosum and the gyms fornicatus are very short. Only 

 in men and apes is the corpus callosum so long that the begin- 

 ning of the gyrus hippocampi lies in the temporal lobe. If you 

 will look at the upper surface of the corpus callosum, you will 

 see on each side of it a thin, gray, longitudinal line (Fig. 18, Lt}. 

 It is the continuation of the atrophic convolution met in the pes 

 hippocampi major, the gyrus dentatus. We call it the stria 

 longitudinalis Lancisi. 



All the convolutions which lie near the margin of the hemi- 

 sphere the gyrus fornicatus, the gyrus hippocampi, the stria 

 longitudinalis Lancisi, and the fascia dentata are very strongly 

 developed in animals having highly perfected organs of smell. 

 In those which, like human beings, have small olfactory lobes, 

 they are somewhat atrophied, and in the dolphin, which has no 

 olfactory lobe, they are totally undeveloped (Zuckerkandl). At 

 the posterior end of the corpus callosum we sometimes see a short 

 convolution which passes in the direction of the fornix and be- 

 comes incorporated with it ; it is the gyrus callosus, which is only 

 present in man as a very atrophic, imperfect,' and inconstant 

 structure. 



On a fresh brain you may see the gyrus unciatus at the 

 apex of the temporal lobe, and from there follow the gyrus hip- 

 pocampi upward. Then observe the prominent arch of the 

 fornix as it passes over the posterior part of the thalamus, and 

 note how it ends in the fimbria, Avhich is visible nearly to the 

 end of the gyrus hippocampi as a white medullary line. 

 Lastly, make a cross-section, which may explain the relations 

 of the foregoing structures. 



On the base of the brain, with the exception of the hippo- 

 campal fissure, which really belongs to the median surface, there 

 are few sulci of importance. On the under surface of the frontal 

 lobe are the orbital and the olfactory fissures. The convolutions 

 between them are regarded as continuations of the frontal con- 

 volutions, and receive the names of the frontal convolutions with 

 which they are respectively continuous. The under surface of 



