954 THE NERVE SYSTEM 



tangential or horizontal fibres, or vertical, the vertical or radial fibres. The tangential fibres 

 run parallel to the surface of the hemisphere, intersecting the vertical fibres at a right angle. 

 They consist of several strata, of which the following are the most important: (1) A stratum 

 of white fibres covering the superficial aspect of the molecular layer; (2) the band of Bech- 

 terew, found in certain parts of the superficial portion of the layer of the smaller pyramidal 

 cells; (3) the external or outer band of Baillarger or the band of Gennari, which runs through 

 the layer of large pyramidal cells; (4) the internal band of Baillarger, which intervenes between 

 the layer of large pyramidal cells and the polymorphous layer. According to Cajal, the tangen- 

 tial fibres consist of (a) the collaterals of the pyramidal and polymorphous cells and of the 

 cells of Martinotti; (6) the arborizations of the axones of Golgi's cells; (c) the collaterals and 

 terminal arborizations of the projection, commissural, or association fibres. The vertical fibres : 

 Some of these viz., the axones of the pyramidal and polymorphous cells are directed toward 

 the central white substance, while others the terminations of the commissural, projection, or 

 association fibres pass outward to end in the cortex. The axones of the cells of Martinotti are 

 also ascending fibres. 



In certain parts of the cortex this typical structure is departed from. The chief of these 

 regions are (1) the occipital lobe, (2) the transtemporal gyres, (3) the hippocampus, (4) the 

 dentate gyre, and (5) the olfactory bulb. 



Special Types of Gray Substance. 1 . The Occipital Lobe. In the cuneus and the calcarine 

 fissure of the occipital lobe Cajal has recently described as many as nine layers. Here the inner 

 band of Baillarger is absent; the outer band of Baillarger or band of Gennari is, on the other 

 hand, of considerable, thickness. If a section be examined microscopically, an additional layer 

 is seen to be interpolated between the molecular layer and the layer of small pyramidal cells. 

 This extra layer consists of two or three strata of fusiform cells, the long axes of which are at 

 right angles to the surface. Each cell gives off two dendrites, external and internal, from the 

 latter of which the axone arises and passes into the white central substance. In the layer of 

 .small pyramidal cells, fusiform cells, identical with the above, are seen, as well as ovoid or star- 

 like elements with ascending axones, the cells of Martinotti. This area of the cortex forms the 

 visual centre, and it has been shown by Dr. J. S. Bolton 1 that in old-standing cases of optic 

 atrophy the thickness of Gennari's band is reduced by nearly 50 per cent. 



2. The Transtemporal Gyres are distinguished by a reduction of thickness of the pyramidal 

 cell layer with closer approximation of the giant cells to each other, while the fusiform cell layer 

 is more deeply situated than elsewhere. This cortical formation is the end station for cochlear 

 nerve projections. 



3. In the Hippocampus the molecular layer is very thick and contains a large number of 

 Golgi cells. It has been divided into three strata: (a) S. convolution or S. granulosum, containing 

 many tangential fibres; (b) S. lacunosum, presenting numerous lymphatic or vascular spaces; 

 (c) S. radiatum, exhibiting a rich plexus of fibrils. The two layers of pyramidal cells are con- 

 densed into one, and these are mostly of large size. The axones of the cells in the polymorphous 

 layer may run in an ascending, descending, or horizontal direction. Between the polvmorphous 

 layer and the ventricular ependyma is the white substance of the alveus (Fig. 930). 



4. The Dentate Gyre. In the rudimentary dentate convolution the molecular layer contains 

 some pyramidal cells, while the pyramidal layer is almost entirely represented by small ovoid 

 elements. 



5. The Olfactory Bulb. In many of the lower animals this contains a cavity which communi- 

 cates through the hollow olfactory stalk with the cavity of the lateral ventricle. In man the 

 original cavity is filled by neuroglia and its wall becomes thickened, but much more so on its 

 ventral than on its dorsal aspect. Its dorsal part contains a small amount of gray and white 

 substance, but this is scanty and ill defined. A section through the ventral part shows it to 

 consist of the following layers from without inward: (1) A layer of olfactory nerve fibres, which 

 are the myelinated axones prolonged from the olfactory cells of the nose, and which reach the 

 bulb by passing through the cribriform plate of the ethmoid bone. At first they cover the bulb, 

 and then penetrate it to end by forming synapses with the dendrites of the mitral cells, presently 

 to be described. (2) Glomerular layer (stratum glomerulosum): This contains numerous sphe- 

 roidal reticulated enlargements, termed glomeruli, which are produced by the branching and 

 arborization of the processes of the olfactory nerve fibres with the descending dendrites of the 

 mitral cells. (3) Molecular layer: This layer is formed of a matrix of neuroglia, embedded in 

 which are the mitral cells. These cells are pyramidal in shape, and the basal part of each gives 

 off a thick dendrite which descends into the glomerular layer, where it arborizes as above, or, on 

 the other hand, interlaces with similar dendrites of neighboring mitral cells. The axones pass 

 through the next layer into the white substance of the bulb, from which, after becoming bent on 

 themselves at a right angle, they are continued into the olfactory tract. (4) AVm- fibre layer: 

 This lies next the central core of neuroglia, and its fibres consist of the axones or afferent processes 

 of the mitral cells which are passing on their way to the brain; some efferent fibres are, however, 



1 Phil. Trans, of Royal Society, Series B, vol. cxciii, p. 165. 



