RECTICULAR FIBRILS PRODUCED BY CAPILLARY ENDOTHELIUM. 91 



The relations of the fibrils to the elements of the glomeruli are naturally of 

 much interest. At favorable points where the epithelial layer of Bowman's capsule 

 is fairly thick, or where it has become separated from the adjacent tissues, as in 

 figure 8, it is seen that the fibrils are related to the capsule exactly as to the thicker 

 epithelium of the convoluted tubules. These fibrils are at some points so far away 

 from capillaries that one hesitates to make the statement that they are produced 

 by endothelial cells here also. In some species wandering connective- tissue cells 

 are found at the point of entrance of the arteriole into the glomerulus, and it is possible 

 that fibroblasts are also present and form the reticulum immediately surrounding 

 the glomeruli. The endothelium of the glomerular tuft is in no case provided with 

 reticulum, affording a marked contrast to that of the intertubular capillaries. 



At first thought it seems impossible that the capillary network, with its open 

 meshes, could he against the renal tubules sufficiently to cover then- surface at all 

 points with a fine network of fibrils, but two considerations remove the force of 

 this objection. (1) Even in thin sections of kidney in which the vascular system 

 has been completely injected with a color-mass, it is found that the capillaries 

 touch the convoluted tubules at practically all points. The familiar figure of an 

 injected human kidney given by Disse (1902, p. 79) illustrates the point. The 

 renal capillary network must be so rich and the individual vessels so flattened 

 against the tubules by the pressure of neighboring tissues as to make the meshes 

 very small and thus practically to complete the surface of endothelium which rests 

 upon the bases of the epithelial cells. (2) Of late we are beginning to think of the 

 capillary endothelial cells not as parts of a rather inert, fixed conducting tube, but 

 rather as dynamic elements, in some organs actively phagocytic, in others perhaps 

 engaged in elaborate chemical processes, at all points ever ready to vary their 

 pattern by putting out and withdrawing sprouts in response to the circulatory needs 

 of the tissues. 



GENERAL CONSIDERATIONS. 



We shall need to await a more complete exploration of the capillaries of the 

 whole body in their relation to the reticulum before attempting to discuss the 

 general significance of the facts here fragmentarily reported. For the present we 

 can merely say that in certain organs where true connective tissue is absent, and 

 the blood-capillaries come into direct contact with actively secreting epithelial 

 cells, the capillary endothelial cells themselves are able to lay down the supporting 

 framework of the gland. Where the support of tissues is provided by fibroblasts 

 and their products, the endothelium seems devoid of reticular fibrils. It will be 

 important to determine next, if possible, whether there are chemical differences 

 between endothelial and fibroblastic reticulum, and whether the endothelial cells 

 which produce fibrils are otherwise different from those which do not possess such 

 a function. 



Already, however, our observations begin to throw light upon certain problems 

 of endothelial physiology and pathology. Since the studies of vital staining demon- 

 strated the phagocytic properties of the cells which line the peripheral sinuses of 



