THE BEAIN OF MAMMALS AND THE OLFACTORY APPAEATUS. 221 



consequence, the cortex of the olfactory area alone remained intact. From this, the 

 olfactory radiation could be traced backward very clearly and distinctly to the corpus 

 mamillare and upward as the taenia thalami to the ganglion habenulse. The fibers 

 must have their origin in the olfactory field itself, for the taenia was not degen- 

 erated, although the fibers had accidentally been badly injured just in front of the 

 ganglion habenjilse on both sides during the operation. 



The olfactorjr fibers have thus been followed, on the one hand, as far 

 as into the cerebral cortex, and, on the other hand, as far as into the ganglion 

 habennlEB and into (?) the corpus mamillare. It will be seen, later, that 

 still other ganglia of the midbrain and interbrain stand in intimate connec- 

 tion with these ganglia. 



In mammals (Loiheissen) fibers from the fornix also mix with those of 

 the taenia thalami, exactly as in reptiles (see above). 



The entire olfactory apparatus thus appears as a huge complex of 

 ganglia and bundles running through the greater part of the brain. In the 

 subsequent chapters its separate parts will be constantly met with. 



It has been seen that a not inconsiderable portion of the surface of the 

 brain owes its perfection essentially to the development of the olfactory ap- 

 paratus. All the gyri and tracts belonging to this are constantly demon- 

 strable in the same place and in a similar relation. 



The development of the remaining portion of the mantle and the fis- 

 sures running in it is much less constant. You will remember that the 

 development of the brain is limited by other factors than those of the 

 cranium, that the presence and the course of the fissuration is determined by 

 the resultant of at least two different developmental tendencies, as has been 

 explained in the preceding chapter. 



Fissures that are deep and long in man may be entirely wanting m 

 closely related animals; others, only suggested in man, are at times well 

 developed in animals. 



In some mammals, the fissura Sylvii, for example, one of the fissures 

 most frequently present, is not present at all or indicated by a shallow sulcus 

 only. The other sulci may assume the most various directions. In general, 

 however, it may be recognized that there are essentially three principal 

 directions: fissures running parallel with the longitudinal fissure, sagittal; 

 fissures curving around the Sylvian fissure, fissures arcuatce ; fissures, finally, 

 of a more or less vertically ascending type, ftssurce coronales. In the human 

 brain the central sulcus is a good example of the last; sagittal sulci traverse 

 the frontal lobe, and arcuate sulci surround the Sylvian fissure in the tem- 

 poral and parietal lobes. It is precisely the vertical fissures which are in 

 most cases but feebly developed in animals. On the bear's brain (Fig. 145) 

 the central fissure is certainly relatively long. Make use of this well-known 



