STRUCTURE OF THE LOBES OF THE CEREBRUM. 401 



the entire structure appears as a knotting up of an olfactory 

 nerve fibre, with some cells interspersed through the mass. 



The peculiar reduction in the quantity of the non-nervous 

 substance that occurs in the brain of Man, and which is here 

 carried to the extreme, though it has already been met with in 

 the general formation of the cortex, and in the cornu Ammonis, 

 thus furnishes an insight into the structure of the olfactory 

 lobes, which it would be impossible to obtain from a considera- 

 tion of those of animals, in consequence of the expansion and 

 enlargement of the curvations of the knot. 



The third layer, Clarke's stratum gelatinosum (r), contains, 

 in its outer zone, scattered, but in the internal zone more closely 

 arranged, nerve cells, that are partly fusiform and partly pyra- 

 midal, and are imbedded in the matrix of the cortex. The suc- 

 ceeding medullary layer of the bulb exhibits a concentric alter- 

 nation of pure medullary layers, with layers of small, closely 

 arranged nerve cells, very similar to the granules of Purkinje 

 of the cerebellum (strata granulosa et medullaria gr, m). These 

 small irregular nerve cells perhaps differ only in point of size 

 from the granule layers of the retina, and from the elements 

 the fourth layer of the entire cortex of the cerebrum. 



The cortex of the olfactory lobe, with the exception of the bulbus, 

 has not as yet been examined with sufficient care to have been made 

 the subject of a monograph. It appears to be composed of a par- 

 ticular form of large cells (perhaps in the manner of the cornu 

 Ammonis). 



The development of the convolution surrounding the corpus callosum, 

 (gyrus fornicatus), which proceeds, part passu, with the olfactory lobes, 

 expresses itself by an increase of the size of the cornu Ammonis, which 

 extends horizontally forward, lying beneath the splenium of the corpus 

 callosum (figs. 238, 241, 242, .4 and Ah), and covering the optic thala- 

 mus, whilst in Man it first commences posterior to these organs. This 

 increase causes the two cornua Ammonis to coalesce with each other 

 in the middle line, and the same thing happens also to the fornix, 

 throughout its whole length. Similarly, in proportion to the develop- 

 ment of the olfactory lobes, the septa pellucida also become thickened 

 and coalesce, with the disappearance of their so-called ventricle, by 

 their median surfaces. The latter coalescence occurs even in the 

 Monkeys (I do not know whether with the exception of the Pri- 

 VOL, II. D I> 



