688 



STRUCTURE OF THE CEREBRUM. 



becomes continuous 

 becomes invested by myelin 



a 



c 



u 11 a (h\ th nnbrancheJ median basilar process, which is an axial cylinder process, and 



] vithX axia? cylinder of a nerve-fibre of the white matter. It ultimately 

 (c) The lateral processes are given off chiefly near the base of 

 the cell, and they soon branch to 

 form part of the ground plexus of 

 fibrils which everywhere pervades a 

 the grey matter. ] ^ 



Each cell is surrounded by a ^ 

 lymph-space, and so are the blood- 

 vessels, in the latter case forming 

 , a perivascular space, which com- 

 I municates with the pericellular 

 lymph-space, as in fig. 476. 

 [Nerve-fibres in the Cortex. 

 ->'/:. The ordinary methods of hardening 



Fig 470 the brain do not enable us to detect 



^S^eSES matter By using W. osrnic 



eating wi A the pericellular acid method, or * eigert s or la Is 



lymph-space b, round the cell method, we obtain such a ^result 



a lymph-space c, containing as is shown in fig. 477. Undo the 



twolymph P corpuscles. x 150. V (P) >f lav q er f ^^Z' 

 J l l tissue (a) devoid of nerve-fibres. 



Beneath it is a layer (6) occupying about the half of the outer 

 layer, which is almost entirely taken up by medullated nerve-fibres ; 

 most of these are fine, but a few of them are coarse, and run parallel 

 to the surface and tangential to the arc of the outer contour of the 

 convolution. Internal to this is a layer of medullated fibres (c), 

 which cross each other in various directions ; while a similar net- 

 work (d) occurs in the small-celled layer. (2) In the layer of large 

 pyramidal cells (3) there are bundles of medullated fibres, running 

 radially (c) ; but at the lower part of this layer there is a very dense 

 network (/), forming (in a Weigert's preparation) a dense, dark 

 band, corresponding to the outer layer of Baillanger. In the layers 

 marked (g and h), which are partly in the third and partly in the 

 fourth cortical layer, the radial arrangement is more marked and 

 more compact, and the thick fibres are more numerous. In the 

 middle is {h) a narrow dense network corresponding to Baillanger's 

 inner layer. The lower part of the fourth layer, and the whole of 

 the fifth, are occupied by i. It is to be remembered, that all the 

 convolutions do not present exactly the same structure and arrange- 

 ment (Oberstcimr).] 



[Variations. The grey matter differs in different parts of the 

 brain. In the grey matter of the cornu ammonis, the large pyra- 

 midal cells of (3) make up the chief mass ; in the claustrum (4) is 

 most abundant. In the central convolutions (ascending frontal 

 and parietal), according to Betz, Mierzejewski, and Bevan Lewis, 

 very large pyramidal cells are found in the lower part of the third 

 layer. Similar cells have been found in the posterior extremities 

 of the frontal convolutions in some animals the posterior parietal 

 lobule, and para-central lobule, all of which have motor functions. 

 In those convolutions which are regarded as subserving sensory 

 functions, a somewhat different type prevails, e.g., the occipital 

 gyri or annectant convolution {B. Lewis). The very large pyramidal cells are absent, while 

 the granule layer exists as a well-marked layer between the layer of large pyramidal cells and the 

 ganglion cell-layer (fig. 475).] 



[Fuchs finds that there are no medullated fibres either in the cortex or medulla until the end 

 of the first month of life. The medullated fibres appear in the uppermost layer at the fifth 

 month, and in the second at the end of the first year, the radial bundles in the deeper layers at 

 the second month. The medullated fibres increase until the seventh or eighth year, when they 

 have the same arrangement as in the adult.] 



[Blood-Veeaelfl. The adventitia of the small cerebral vessels contains pigment and granular 

 cells, filled with oil -granules. In the new-born child, the blood-vessels of the brain are beset 

 with cells, filled with fatty granules. Perhaps the granules supply part of the material for the 

 formation of the myelin sheath on the nerve-fibres. About the fifth year, the fat is replaced 

 by a yellow pigment. In adults, yellow or brown glancing pigment-granules are found in the 



Fig. 477. 

 Vertical section of a frontal 

 convolution (Weigert's 

 method) x 50. P, pia 

 mater ; 1-5, five layers of 

 Meynert ; a, superficial 

 layer of connective-tis- 

 sue ; b, i, successive layers 

 of medullated nerve - 

 fibres : k, white matter. 



