146 ANATOMY OF THE CENTEAL NEEVOUS SYSTEM. 



amphibians and reptiles, in birds and mammals. The development of the 

 mantle is of especial interest. From a small beginning in teleosts it develops 

 into the enormous organ which we recognize in man as the cerebral hemi- 

 spheres, and with this development progresses the capacity for the higher 

 psychical activities. 



Pig. 20 shows well the separate parts of the embryonic human brain, 

 which may serve here as a prototype, since in the depicted one only struct- 

 ures appear which are constant in their recurrence. Study, also. Fig. 55, 

 and note the varying development of the mantle shown in the four sagittal 

 sections given. 



I. THE OLFACTOEY APPAEATITS AND THE COEPUS STEIATUM. 



These structures, together with the spinal cord, cerebellum, and mid- 

 brain, show, through the whole series, little essential diiierence. 



Except the Ggl. habenulse and optic system, no other portion of the 

 brain is so constant in structure as the olfactory apparatus. Only the rela- 

 tive size varies; the structural features remain unchanged. For our knowl- 

 edge of the structure we are especially indebted to S. R. y Cajal, Van 

 Gehuchten, Kolliker (and Edinger). 



From the epithelium of the nasal mucous membrane (Fig. 16, a) long 

 terminal fibrillae run backward. They are called Fila olfactoria, and pass 

 through the cribriform plate into the cranial cavity. 



Within the cranium they reach, after a longer or shorter course (accord- 

 ing to the class), the anterior end of the brain, where they enter within it. 

 The whole bundle, which may be subdivided, is called the Nervus olfac- 

 torius} 



The Fila olfactoria pass to an anteriorly directed evagination of the 

 forebrain-vesicle. This evagination forms on the base of the brain a more 

 or less elongated tube which, in most animals, remains hollow. This tube 

 is called Lobus olfactorius anterior. From the place where it passes into the 

 base of the brain begins the posterior olfactory region, which in mammals 

 is called the Lobus olfactorius posterior. Since in lower vertebrates the 

 homology is not yet certain, we will designate the anterior simply as Lobus 

 olfactorius, and the posterior, which includes the whole base of the brain, 

 as area olfactoria. 



At the place where the Fila olfactoria enter the anterior end of the 

 olfactory lobe they break up, sometimes, after decvissation and exchange of 

 fibers, into very fine terminal ramifications. These enter the apex of the 

 lobe, where they meet the terminal ramifications of cells which lie in that 



' The term Ttadi.r olfactoria would be a better one, since Figs. 15 and 16 show 

 the Fila olfactoria and the ''olfactory nerve'' to be homologous to the roots of the 

 spinal and of most of the cranial nerves, being the neuraxons of the neurons involved. 



