540 ^- ^- Andriezen. Un a system of fibre-cells. 



matter, and to a less extent the white matter, while it admirably 

 serves by its textme and porosity to allow of the fi-ee passage of 

 lymph and metabolic products which enter into tlie fluid and general 

 metabolism of the nerve-centres. 



Hence the condition of the felted sheath must assume a corre- 

 sponding importance in the study of the problems of brain pathology, 

 both fi'om the standpoint of defective evolution of such a structure, 

 and in the incidence of disease in which vascular, perivascular, and 

 adjacent structures are the fii-st and earliest to suffer. A future com- 

 munication will deal with its embryological development, and latter 

 on with its changes in the brains of the insane. 



Explanation of PI. XXI. 



Fi^. 1. A small blood vessel of the Human Brain, showing several small (in- 

 trinsic) neuroglia cells on its surface, and a few larger and remote 

 extrinsic cells. Staining with Silver Chromate. 



Fig. 2. Several neuroglia fibre-cells (intrinsic) forming a sheath (perivascular) 

 of fibres interwoven into a feltwork. a an encircling cell; all the other 

 are elongate cells, b a fibre entering the sheath at right angles from a 

 distant (extrinsic) cell. Chromate of silver staining. 



Fig. S. a A vessel indicated by dotted lives. An elongate cell is seen giving 

 longitudinal and oblique fibres (intrinsic); also several (extrinsic) fibres 

 entering the sheath at right angles (Humau Brain), b Cross section of 

 a vessel (dotted), surrounded by a transverse cell (intrinsic). A few 

 fibres are seen entering the feltwork perpendicularly. 



Fig, 4. A perfectly stained elongate Glia-cell lying apparently along a vessel 

 of the Human Brain imed. subst.}. Silver Chromate staining. 



Fig. .5. Oblique section of a perivascular felted sheath of a median sized vessel 

 (Human Brain). Showing the longitudinal, trausverse and oblique fibres 

 contributed to it by intrinsic cells of the sheath, and the larger extrinsic 

 cells (b). Chromate of suver staining. 



Fig. 6. A large vessel showing perivascular felted-sheath, very dense on the 

 right side, less dense on the left side (a) where it is distinctly separated 

 by a space from the blood vessel proper and its contacts. Human Brain 

 (medullary substance). Chromate of Suver Staining. 



Fig. 7. Two caudate glia cells forming the (Fig. 7) surface feltwork of l^t layer 

 (top of convolution: Human Brain) and one cell situated a little deeper. 

 Dotted line drawn between the 1st layer and 2n'i layer of the cortex. 

 Chromate of Silver Staining. 



*0' 



