If) 



Mr. W. B. Lewis. On the 



[Nov. 17, 



(b.) The central projections of the olfactory organ. 



The cortex of the cerebrum of the rabbit and rat is naturally divi- 

 sible into two distinct segments, which (to follow the nomenclature 

 advocated by Broca) may be termed the great limbic lobe and extra- 

 Jimbic mass or parietal segment. The great limbic lobe is further 

 divisible into — 



(a.) An upper limbic arc (gyrus fornicatus). 



(&.) A lower limbic arc (gyrus hippocampi). 



(c.) An anterior limbic arc (olfactory lobe). 



Minute examinations of these regions reveal the presence of eight 

 diverse types of cortex, which are distinguished from each other by the 

 number or kind of the constituent layers. Meynert enumerates only 

 five types as distinguishable in the cortex of human brain ; and since 

 his Sylvian type differs in degree rather than in kind, and should 

 be therefore eliminated, we find in the small brain of the Rodent 

 where frontal, temporal, and occipital lobes are absent, a greater variety 

 of cortical constitution than what Meynert assigns to man. Meynert's 

 enumeration, however, falls far short of the truth, since all the types 

 found in the hemispheres of these lower animals prevail also in human 

 brain, which therefore would embrace, at the very least, ten distinct 

 types of cortical lamination. 



The eight formations occurring in the rabbit and rat are as 

 follows :— 



1 . Type of upper limbic arc. 



2. Modified upper limbic type. 



3. Outer olfactory type. 



4. Inner olfactory type. 



5. Modified lower limbic type. 



6. Extra-limbic type. 



7. Type of cornu ammonis. 



8. Type of olfactory bulb. 



The first six may be best summarised by noting the area covered by 

 each, and the special feature bestowing upon each its typical character. 



1. Type of Upper Limbic Arc. — Area. This cortex covers the whole 

 anterior two-thirds of the upper limbic arc, reaching from frontal pole 

 to near the posterior border of the corpus callosum. In front of the 

 corpus callosum it spreads outwards over the exposed vertex, spanning 

 the frontal end of the hemisphere here. — Type. A four laminated 

 cortex, as follows: — 1. A peripheral cortical zone. 2. Layer of small 

 pyramidal cells. 3. Ganglionic cell layer. 4. Layer of spindle cells. 

 The angular cells forming the second layer in human brain are here 

 absent. The ganglionic formation is clearly identified with that of 

 higher animals. Like the latter, they are arranged in clustered 

 groups or solitary file, and are subject to the interposition betwixt 

 them and the small pyramids of a layer of angular or granule cells in 



