STRUCTURE OF THE LOBES OF THE CEREBRUM. 397 



form the vessels of the white substance of the trough-like 

 layer (alveus, Muldenblatt, A A). In this layer the apical 

 processes of the pyramids unite to form a nervous plexus. 



3. The external half of the third layer of the cortex, the 

 medium-sized pyramids of which are not here present, is 

 represented, as Kupffer has remarked, by a region marked with 

 parallel striae, by the long apical processes of the pyramids 

 (stratum radiatum). 



Succeeding to this, the pyramids of the inner half of the 

 third layer, arranged in numerous tiers, form the proper comu 

 Ammonis formation (c). The lacunar spaces occurring around 

 the nerve corpuscles, which have been attributed to the re- 

 traction of the connective tissue consequent on the hardening 

 of the preparation, are particularly visible in these large types 

 of cells, and have been regarded by Obersteiner as pericellular 

 lymph passages. He injected them, and, on making sections, 

 found them to communicate with perivascular spaces, and to 

 include corpuscles that he believed to correspond with unde- 

 veloped forms of lymphoid elements. 



The fourth and fifth layers of other parts of the cortex are 

 not represented in the cornu Ammonis of Man by the presence 

 of any non-nervous substance, though in animals (Cats and 

 Rabbits) there is certainly a layer of it, constituting Kupfier's 

 stratum moleculare. Hence it was that Kupffer failed to see 

 the immediate connection of the alveus with the pyramids in 

 Man, this being effected mediately, through the stratum mole- 

 culare, in other animals. The alveus (A) represents the medulla 

 belonging to the cortex of the cornu Ammonis, and, like the 

 rest of the medullary layers of the hemisphere, is invested 

 upon its surface by the epithelium of the lateral ventricles. 

 The epithelium of the ventricle of the olfactory lobe has been 

 described by Clarke as of an elongated columnar form, identical 

 with that of the central canal of the spinal cord, the cells termi- 

 nating externally in fibres that communicate with granules 

 which Stilling alone considers to be nervous. I regard this 

 form of epithelium as common to all cerebral ependyma, and 

 have satisfied myself on this point in regard to the surfaces 

 of the corpora striata, optic thalami, and corpus callosum, 

 whilst Gerlach has established its presence in the aqueduct. 



