680 THE NERVOUS SYSTEM. 



of branches, which are directed toward the surface of the cortex ; (6) the cells of 

 Martinotti, which are chiefly found in the polymorphous layer. Their dendrites 

 are short, and may have an ascending or descending course, while their axons pass 

 out into the molecular layer and form an extensive horizontal arborization. 



Nerve-fibres. These fill up a large part of the intervals between the cells, and 

 may be medullated or non-medullated the latter comprising the axons of the 

 smallest pyramidal cells and the cells of Golgi. In their direction the fibres may 

 be either transverse (tangential or horizontal) or vertical (radial). The transverse 

 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 external band of Baillarger. or band of Gennari, 

 which runs through the layer of large pyramidal cells ; (3) the internal band of 

 Baillarger, which intervenes between the layer of large pyramidal cells and the 

 polymorphous layer. According to Cajal, the transverse fibres consist of (a) the 

 collaterals of the pyramidal and polymorphous cells and of the cells of Martinotti ; 

 (b) the arborizations of the axons 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 axons of the pyramidal and polymorphous cells, 

 are directed toward the central white matter, while others, the terminations of the 

 commissural, projection, or association fibres, pass outward to end in the cortex. 

 The axons 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 hippocampus major, (3) 

 the dentate convolution, and (4) the olfactory bulb. 



Special Types of Gray Matter. 



1. 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 (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 axon 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 cells 

 with ascending axons (cells of Martinotti). This area of the cortex forms the 

 visual centre, and it has been shown by Dr. J. S. Bolton J that in old standing 

 cases of optic atrophy the thickness of Gennari's band is reduced by nearly 

 50 per cent. 



2. In the hippocampus major the molecular layer is very thick and contains a 

 large number of Golgi cells. It has been divided into three strata : (a) S. con- 

 volutum or S. granulosum, containing many tangential fibres ; (ft) S. lacuriosum, 

 presenting numerous lymphatic or vascular spaces ; (c) S. radiatum, exhibiting a 

 rich plexus of fibrils. The two layers of pyramidal cells are condensed into one, 

 and the cells are mostly of large size. The axons of the cells in the polymorphous 

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

 polymorphous layer and the ventricular ependyma is the white substance of the 

 alveus. 



3. In the rudimentary dentate convolution the molecular layer contains some 

 pyramidal cells, while the layer of pyramidal cells is almost entirely represented 

 by small ovoid cells. 



4. The Olfactory Bulb. In many of the lower animals this contains a cavity 

 which communicates through the hollow- olfactory stalk with the cavity of the 



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



