20 Comparative Structure of the Brain in Rodents. [Nov. 17, 



the fine non-fasciculated nerve fibrils which originate in the granule 

 layers of the bulb. A small medullated band for this olfactory area 

 is also derived from the inner margin of the superficial olfactory 

 fasciculus. An important connexion is established with, the motor 

 columns of the cord by fibres passing through this region. This 

 double connexion of the olfactory organ with the cord and cerebrum 

 is the more frequent occurrence in Mammalia. In this olfactory area 

 is an extensive system of arciform medulla, which runs parallel with 

 the cortex at the base destined to reach the under aspect of the 

 callosal commissure through the structure of the septum lucidum. 



3. Arciform Medulla. — -The more important fasciculi, after piercing 

 the callosal commissure, run as a longitudinal band upon the upper 

 surface of that commi .sure, just before the latter spreads outwards 

 to the cortex of the vertex. Its destination is to the formation of the 

 modified upper limbic type, internal to the primary parietal sulcus. 



At the occipital extremity of the hemisphere, therefore, there are 

 two important regions characterised as follows: — 



a. Modified upper limbic coi tex. containing a deep belt of granule 

 cells ; two well-defined intracortical arciform stripes connected with 

 the cornu ammonis, and the terminal expansions of the olfactory 

 arcuate system. 



b. Modified lower limbic type presenting the large inflated cell-forma- 

 tion ; two similarly connected arcuate stripes ; and lastly the terminal 

 expansion (in the rat) of the central olfactory fasciculus after its 

 decussation in the anterior commissure. 



4. Superficial Olfactory Fasciculus. — Otherwise termed the outer 

 olfactory root, becomes rapidly attenuated by the termination of its 

 fibres all along its course in the cortex of the anterior and inferior 

 limbic arcs. It has also been stated above that a large fasciculus from 

 its inner margin is distributed to the deep olfactory medulla. 



5. Taenia Semicircular is. — This arciform band, arising in the cortex 

 of the gyrus hippocampi, and closely following the curved inner 

 surface of the caudate nucleus, is stated by many authorities to 

 terminate in the descending pillar of the fornix. This fact I have 

 been unable to confirm as regards the rabbit and rat, but there are 

 certainly two clearly defined fasciculi which descend from the taenia 

 semioircularis, one to arch round into the anterior commissure, pro- 

 bably decussating here ; the other or deeper set of fibres enters the 

 olfactory area. Hence this arcuate band brings the cortex of the 

 gyrus fornicatus into relationship with the olfactory area of its own 

 side and into crossed relationship with the olfactory bulbs, unless the 

 fibres of the taenia entering the anterior commissure are purely com- 

 missural in their character. 



The Corpus Striatum and its Cortical Connexions. — The following 

 conclusions have been arrived at after a careful study of micro- 



