174 THE CEREBRAL HEMISPHERES. 



ance to the layer they occupy (stratum granulosuni). Superficial to this again is a 

 well-marked layer of medullated fibres continuous with the reticulated white substance 

 of the unci.nate convolution (see p. 156). It is known as the involuted medullari/ 

 lamina (fig. 124, 6), and represents an increased development of the thin layer of 

 white fibres which is ordinarily found in the molecular or superficial layer of the 

 grey matter. 



In the fascia dentata the large pyramidal cells are placed in the centre and are 

 irregularly arranged : they are surrounded by an incomplete ring of closely packed 

 small pyramidal cells (stratum granulosum, fig. 124, *), outside which is a very 

 broad superficial layer of neuroglia with a few scattered cells. 



Minute structure of the olfactory lobe. The peculiar structure of this part 

 of the brain can best be understood by a reference to its mode of development. It 

 is formed as a hollow outgrowth from the vesicle of the cerebral hemisphere (after- 

 Fig. 125. SECTION ACROSS THE MIDDLE OF THE OLFACTORY TRACT 

 (Henle). 



v, ventral surface ; d, dorsal ridge. From without in are seen 

 successively: (1) a thin superficial layer of neuroglia; (2) a (darkly 

 shaded) layer of transversely-cut medullary fibres, of very unequal 

 thickness in different parts ; (3) the central grey matter projecting up 

 into the dorsal ridge and here and there extending to the surface and 

 partially interrupting the medullary layer. 



wards the lateral ventricle), and in most of the lower 

 animals (in which it is much more developed than in 

 man), it exhibits even in the adult condition a central 

 cavity (lined with ciliated epithelium), and in some, as 

 already stated, this retains throughout life its connec- 

 tion with the lateral ventricle. The walls of the hollow outgrowth become 

 thickened and differentiated into a central layer of neuroglia next to the cavity, 

 a well-marked intermediate layer of white substance outside this, and a peripheral 

 layer of grey matter surrounding the whole. In man and apes the same changes 

 occur, but the cavity becomes completely obliterated and in its stead we find nothing 

 but central neuroglia, which forms for the most part a tract flattened out laterally, 

 and containing but few cells. The white or medullary substance around this appears 

 in section in the form of a flattened ring consisting of longitudinal white fibres 

 (fig. 125). In the olfactory tract the peripheral layer of grey matter is very thin 

 and inconspicuous, so that the white substance almost everywhere shows through it, 

 except along the dorsal ridge where there is an accumulation of the grey substance, 

 extending into and partly interrupting the medullary ring (fig. 125, d). In the 

 bulb on the other hand this dorsal accumulation of grey matter is not seen ; but 

 upon the ventral side of the flattened medullary ring (fig. 126, 1, 2, 3) in place of 

 the thin scarcely visible layer of grey substance in the corresponding situation in 

 the tract, a thick layer of grey matter is found and forms indeed the greater part of 

 the thickness of the bulb, what was originally the central cavity being consequently 

 now placed near the dorsal surface. This grey matter as seen in section exhibits 

 the following parts (fig. 126, 4 to 8) : 



(1.) A granule layer of considerable thickness (fig. 126, 4, 5, 6) lying next to the 

 medullary ring, and characterised by the presence of numerous small cells, like those 

 found in the deeper or granule layer of the grey cortex of the cerebellum. The 

 layer is not entirely composed of these cells however, for there are present in addition 

 a number of reticulating bundles of medullated fibres which separate the " granules " 

 into groups, and other medullated fibres which pass vertically between the medullary 

 ring and the next layer. There are also a number of large nerve-cells, having for 

 the most part a conical shape (mitral cells, fig. 127, m.c.), the axis-cylinder processes 



