788 THE NERVOUS SYSTEM. 



amidal cells in the third layer of the cortex. The collaterals, already referred to, 

 of these processes become medullated and form two plexuses, one along each 

 border of the third layer. These plexuses appear to the naked eye as two fine 

 white lines (Baillarger) in sections of the cortex of a fresh brain. 



Special Types of the Gray Matter of the Cortex. The special types of gray 

 matter of the cortex are the following: 



(1) On the posterior portion of the occipital lobe, near the calcarine fissure, 

 the gray matter consists of six or eight layers. This is produced by the inter- 

 calation of intermediate small, irregular cells, similar to those forming the fourth 

 layer of the typical cortex. Furthermore, the large pyramidal cells of the typical 

 third layer are very few, while, on the other hand, in the upper part of the ascend- 

 ing frontal convolution (psycho-motor region) these pyramidal cells of the third 

 layer are, many of them, of unusual size. 



(2) In the gray matter of the cortex of the hippocampus major or cornu 

 Ammonis pyramidal cells are found, such as have been described in the third 

 layer of the typical cortex. They constitute the greater part of the structure, 

 the fourth and fifth layers being absent. Hence this layer is called the formation 

 of the cornu Ammonis. The bases of these cells are close under the white 

 lamina (alveus) which covers the hippocampus on its ventricular aspect. The 

 second layer i. e. toward the hippocampal fissure contains no cells. It is 

 represented by a closely interwoven arborization of the dendrites (protoplasmic 

 processes) of the pyramidal cells just mentioned, of which the axis-cylinder pro- 

 cesses pass, in the opposite direction, into the alveus. Finally, beyond the 

 second layer is the first layer of the gray matter of the hippocampus, or, as it 

 is termed, the granular formation (Meynert), and consists of numerous small, 

 irregular cells, which resemble the nerves-corpuscles found in the internal granule- 

 layer of the retina. 



(3) In the Sylvian fissure the fifth layer of the cortex contains an unusual 

 number of fusiform cells ; hence this layer, in this region, is called the " claus- 

 tral formation," because of the number of the same kind of cells in the structure 

 of the claustrum. 



(4) In the olfactory bulb, which is a portion of the cerebral hemispheres, form- 

 ing "a cap superimposed upon a conical process of the cerebrum," is another 

 variety of structure, differing from the type of the cortex of the hemispheres. 

 The bulb consists of both gray and white matter, and in most of the lower animals 

 retains a central cavity lined by epithelium, around which is a layer of neuroglia, 

 surrounded in its turn by white fibres, the whole being enclosed by gray matter. 

 In man the central cavity is obliterated, and in the " centre " of the bulb is found 

 neuroglia surrounded in section by a flattened ring (medullary ring) of white 

 fibres. The gray matter is now exceedingly thin dorsally, but very thick ven- 

 trally, and in section this ventral portion shows the following layers from below 

 upward : 1. The olfactory nerve-layer, consisting of a plexus of non-medullated 

 nerve-fibres derived from the nerves which supply the olfactory region. These 

 fibres pass downward through the foramina in the cribriform plate of the ethmoid, 

 and dorsally into the glomeruli of 2, the stratum glomerulosum, consisting of 

 nodulated masses (the glomeruli), each mass consisting of a dense interlacement 

 of fibres, which are partly the prolongations of the olfactory fibres just mentioned, 

 and partly the dendrites of the mitral cells in the superjacent part of the next layer. 

 Small neuroglia-cells also are found in these glomeruli. 3. The granular layer, 

 consisting of (a) small irregular nerve-cells resembling those of the granule-layer 

 of the cortex of the cerebellum ; (b) a deeper layer (next to the stratum glomeru- 

 losum) of large, conical cells (mitral cells). The dendrites of these pass down to 

 the glomeruli (see above), while their axis-cylinder processes (medullated) pass 

 upward between small cells of the granule-layer to the medullary ring, with the 

 fibres of which, after bending sharply backward, they become continuous, and 

 thence pass backward along the olfactory tract toward the base of the brain ; that 

 is, the fibres of the medullary ring are the continuations of these processes. 



