RETICULAR FIBRILS PRODUCED BY CAPILLARY ENDOTHELIUM. 89 



was secured, in the preparation of some of the specimens illustrated in this paper, 

 either by selecting engorged tissues, by ligating the vena cava under anesthesia, 

 or (in the case of the fetal kidney) by injecting oxalated blood of the same species 

 into the arterial system under considerable pressure. 



The test of our hypothesis is shown in figures I 1 and 2, which display the 

 condition found in the fully developed corpus luteum of the cow and sow respective- 

 ly. There can be no doubt that in this organ the endothelial cytoplasm itself con- 

 tains the fibrils of reticulum which support the tissues by embracing each lutein 

 cell in their basket-like meshes. The delicate complexity of the network, well seen 

 in figure 2, is explained by the richness of the capillary bed, which touches every 

 cell in the whole gland. As in the figures, the fibrils rarely if ever embrace the 

 endothelial nuclei, but usually pass between them and the lutein cells, leaving the 

 nuclei to bulge into the capillary lumen. Fibrils are never seen in the endothelium 

 of vessels whose walls are more than one cell-layer thick, but such arterioles and 

 venules are provided with a perivascular reticulum. 



ADRENAL. HYPOPHYSIS. THYROID. 



The adrenal gland is another organ whose framework is known to consist of 

 reticular fibrils (Flint, 1900), yet contains no fibroblasts. Professor Evans's studies 

 of vital staining with benzidene dyes provide a delicate test for the presence of fixed 

 as well as wandering connective-tissue cells, and they show that there are no fibro- 

 blasts in the adrenal cortex (personal communication). Figure 3 illustrates the 

 condition shown by the Bielschowsky method in the zona reticularis of the rat's 

 adrenal; figure 4, in the zona fasciculata. Here again there can be no doubt that 

 it is the capillary endothelium that subserves the function of reticulum formation. 



In the anterior lobe of the hypophysis the relation between the circulating 

 blood and the epithelial cells is so close that not only is there no space for fibro- 

 blasts, but some have even doubted the continuity of the capillary tubes, suggesting 

 incomplete walls, as in the liver. In this gland the fibroblastic activity of the 

 endothelium is very readily demonstrated, for it is seen in the walls of relatively 

 large sinusoids (fig. 5). 



In text-book descriptions of the thyroid gland it is stated or implied that the 

 reticular framework, mentioned by Mall (1891) and described by Flint (1903), is 

 laid down by connective tissue in interfollicular strands which carry the blood- 

 vessels; yet careful study of thin sections of the thyroid readily confirms the state- 

 ment of Flint that "interfollicular connective tissue is scant save in the neighbor- 

 hood of the great vessels." Major (1909) has pointed out the exceedingly close 

 contact between the perifollicular capillaries and the follicle-cells. In preparations 

 of the rat's thyroid, made to illustrate this paper, the strands of connective tissue 

 along the larger vessels are composed of collagenous fibers, with only here and there 

 a cell which may be interpreted as a fibroblast containing reticular fibrils. Away 

 from the arteries and veins, between the follicles, the only cellular elements present 



1 Figure 1 is from a preparation of Mr. J. F. Cobb, for whose assistance, cut short by his entrance into the military 

 service, I am much indebted. 



