STRUCTURE OF NERVOUS TISSUE. 339 



cylinders of tubular fibres, or with gelatinous fibres. Such vesicles are seen 

 alike in the ganglionic masses of the Cerebro-spinal, and in those of the Sym- 

 pathetic system. Besides the finely-granular substance just mentioned, these 

 cells usually contain a collection of pigment-granules, which especially cluster 

 round the nuclei, and give them a reddish or yellowish-brown color. This pig- 

 ment seems to have some resemblance to the hsematine of the blood; and it is 

 usually, if not invariably, deficient among the Invertebrata, as well as less 

 abundant in Reptiles and Fishes. The vesicles are sometimes covered with a 

 layer of a soft granular substance, which adheres closely to their exterior and 

 to their processes ; this is the case in the outer part of the cortical substance of 

 the human brain. In other instances, each cell is inclosed in a distinct enve- 

 lop, composed of smaller cells, closely adherent to each other and to the con- 

 tained cell; such an arrangement is common in the smaller ganglia, and in the 

 inner portion of the cortical substance of the brain. The diameter of the 

 vesicles is extremely variable, owing to the changes of form above described ; 

 that of the globular cells is usually between l-300th and l-1250th of an inch. 



341. In the central or ganglionic masses of the Nervous system we find these 

 vesicles aggregated together, and imbedded in a finely granular matter ; the 

 whole being traversed by a minute plexus of capillary bloodvessels. The entire 

 substance, made up of these distinct elements, is commonly known as the cine- 

 ritious or cortical substance ; being distinguished by its color, in Man and the 

 higher animals at least, from the white substance composed of nerve-tubes, of 

 which the trunks of the nerves, as well as a large part of the brain and spinal 

 cord are made up ; and occupying in the brain a position external to the latter, 

 which is often termed the medullary substance. This position, "however, is 

 quite an exceptional one ; for in the spinal cord and in the scattered ganglia of 

 Vertebrated animals, and in all the ganglionic centres of Invertebrata every- 

 where, in fact, except in the Brain the vesicular nerve-substance occupies the 

 centres of the ganglia; consequently, the terms " cortical" and " medullary/' as 

 applied to the vesicular and tubular substances respectively, are quite inappro- 

 priate. Nor are the designations that have reference to their color, much more 

 uniformly correct : for, as we have seen, the vesicular substance may be destitute 

 of internal pigment-granules, and the blood in its capillary plexus may be pale 

 or colorless ; so that the reddish-gray hue, which is expressed by the term 

 " cineritious," may be entirely wanting; 'whilst, on the other hand, we have 

 seen that certain of the nerve-fibres, making up what is commonly termed the 

 " white' ' substance are of a gray color. Hence the only valid distinction be- 

 tween these two kinds of nervous matter, is that which has reference to their 

 constitution ; as consisting of cells or vesicles on the one hand, or of tubes or 

 fibres on the other. 



342. The connection between the "fibrous" and the "vesicular" elements, in 

 the Nervous Centres, is a question to which great attention has been paid for 

 some time past by Microscopic observers; but it is one of such intricacy and 

 difficulty, that it is still far from being satisfactorily elucidated. It is certain, 

 however, that some of the nerve-fibres come into direct continuity with the 

 ganglionic cells or vesicles ; and two principal modes of such connection have 

 been observed. Some of the fine granular prolongations of the " stellate" nerve- 

 cells, are traced into the form of "gelatinous" fibres; and these, at a greater 

 distance from their centre, exhibit more and more of the tubular character, and 

 at last become ordinary ' tubular" fibres. On the other hand, a spheroidal 

 vesicle is often observed to be directly continuous with a tubular fibre, the 

 transparent membranous envelop of the latter being a prolongation of the cell- 

 wall, and the substances which it contains being in connection with those in- 

 cluded within the cell-cavity (Fig. 107, c, and 108). According to Prof. Kbl- 



