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HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



coat and the mucosal venules was rapid and unidirec- 

 tional. Backflow of blood was observed in veins of the 

 submucosal plexus. 



.1//. rot initiation in the Bulbar Conjunctiva 



The bulbar conjunctiva is a highly vascularized 

 transparent mucous membrane on the anterior surface 

 of the eye. It extends from the palpebral conjunctiva, 

 which lines the eyelid, to the cornea. The pattern of 

 its blood vessels were derived primarily from studies 

 by Grafflin & Corddry (56) and Lee & Holze (77). 



A description of the arrangement of the superficial 

 blood vessels of the human conjunctiva is given by 

 Lack et al. (74) in a study designed to observe vascular 

 changes in hypertension. Arterioles were seen to 

 divide into numerous side capillaries and terminate 

 with an end capillary. The term "end capillary" is 

 not defined. The end capillary on occasion functioned 

 as a "through-and-through" channel. The capil- 

 laries appeared to be uniform in caliber. Only rare 

 arteriovenous anastomoses of the short type were 

 seen. 



A more detailed description appears in a paper by 

 Lee & Holze (77) in 1950, in which they state that 

 the arrangement of terminal arterioles, capillaries, 

 and venules in the human conjunctiva was in accord 

 with the pattern of vessels as seen in the omentum 

 and mesentery of other animals, referring to the de- 

 scriptions of Chambers & Zweifach (20) and Lee & 

 Lee (78). Capillaries arose at intervals from end ar- 

 terioles to form an irregular network of vessels which 

 then rejoined to form the venular system. The ar- 

 terioles were also seen to terminate in main channels 

 which communicated directly with a venule. This 

 pattern was most often seen at the corneoscleral 

 junction. It was also noticed that blood continued to 

 flow from arterioles to venules, through the patent 

 arteriovenous channels, at a time when there was 

 widespread arteriolar and precapillary constriction. 

 The precapillaries were found to be more sensitive to 

 stimuli than their parent arterioles. 



Observations of blood flow revealed active contrac- 

 tion of vessels. Constriction of precapillaries was seen 

 to occur at their point of origin from the parent ar- 

 teriole. Attention was directed to these precapillary 

 sites because of the difficulty in determining minor 

 changes in diameter or flow in the arterioles. Com- 

 plete constriction occurred at the precapillary region 

 lasting for 2 to 3 min. After a gradual relaxation, 

 blood flow continued for 1 to 5 min before the next 

 constriction. The periods of constriction and relaxa- 



tion were found to be very irregular, with relaxation 

 predominating. 



Arteriolar flow was rapid, capillary flow was slower, 

 and also intermittent due to spontaneous changes in 

 diameter at the precapillary sites, while venous flow 

 speeded up after entering the system of collecting 

 venules and was consistently regular. 



The proposal of a definite structural and functional 

 unit, such as the preferential channel, as described by 

 Chambers & Zweifach (20) in the rat mesentery and 

 seconded by Lee & Holze (77) in the human conjunc- 

 tiva, prompted Grafflin & Bagley (55) to reinvestigate 

 the human conjunctiva. These investigators were im- 

 pressed by an endless variety of vascular patterns 

 with no apparent plan of organization. This paper 

 was followed by one by Grafflin & Corddry (56) who 

 reinvestigated, with improved equipment, the archi- 

 tecture of vascular beds in the human conjunctiva 

 in an effort to resolve the differences between the 

 earlier observations and those of Lee & Holze (77). 

 Once again they reported a great variety of vascular 

 patterns with the lack of any recognizable structural 

 and functional unit similar to that proposed by 

 Chambers & Zweifach (20). They saw, however, 

 vessels between arterial and venous channels that 

 were larger than capillaries. They believed that these 

 vessels were arteriovenous communications with a 

 functional significance different from that of capil- 

 laries. They do not say what the difference is. The 

 arteriovenous communications were seen so fre- 

 quently that the investigators believed that they were a 

 characteristic feature of the conjunctival vascular 

 beds. In freehand drawings at magnifications up to 

 80 times, a variety of vascular patterns are shown. 

 The arteriovenous communications, veno-venous 

 anastomoses and arterio-arterial anastomoses are 

 common features (fig. 12). Although at first glance 

 the vascular pattern may seem very complex, it is 

 comparable in its arrangement to other terminal 

 vascular beds which have been presented in such 

 detail covering a large area. A smaller area is seen 

 in figure 13. 



A representative type of arteriovenous anasto- 

 mosis, as seen in vascular beds below the surface of 

 the conjunctiva (presumably on the episcleral sur- 

 face), is shown in figure 14. It bears a striking re- 

 semblance to both photomicrographs and diagrams 

 of the vascular bed in the rat mcsoappendix. The 

 authors do not describe the kind of blood flow through 

 these vessels which would qualify them as preferential 

 channels on a functional basis. 



The vascular patterns presented by these authors 



