Blood vessels of the bulbar conjunctiva in man.

LARGELY owing to technical difficulties the role of the capillary in human disease is not known. Chambers (1948) stated that an adequate survey of the capillary bed for study of the variations in its functional activity required tissue in which the vessels could be traced from the arteriolar to the venular ends, and at the same time be exposed for observation with a minimum of trauma so as not to upset the normal activity of the functioning components of the bed. In man, the only available site which fulfils these requirements is the bulbar conjunctiva. The topography of minute vessels was greatly clarified by Chambers and Zweifach (1944), who demonstrated the existence of a structural unit in the architecture and physiological activity of the capillary bed. These investigators found, in the mesentery of the dog and the meso-appendix of the rat, occasional vessels of capillary dimensions through which the blood was flowing more rapidly than through the capillaries adjacent to them. These vessels originated as branches or as prolongations of the terminal arterioles and could be traced through the vascular bed until they merged with similar channels to form the venules. These thoroughfare or central channels were frequently varicose along their proximal portions. The capillaries were branches of the central channels. Along the proximal portion of a central channel the capillaries branched off more or less abruptly and carried the blood away from it. Along the distal portion they joined at increasingly acute angles in the direction of the flow within the channel. The junctional segments of the capillaries leaving the meta-arteriole were designated the pre-capillaries, and were encircled by contractile muscle cells, the pre-capillary sphincters. The muscle cells of the junctional segments along the remainder of the central channel progressively diminished until, at the venular portion, they were absent. The true capillaries were purely endothelial vessels which extended between the pre-capillaries and the capillaries which led into the venous channels. Capillaries were not branches only of the central channels. Along the length of a typical arteriole occasional capillary offshoots were seen.