PATTERNS OF THE A-V PATHWAYS 



9" 



as well as in the venous sinuses, and assigned this 

 normal activity of vascular smooth muscle to ''physio- 

 logical sphincters." No significant differences were 

 noted in the structure or activities of living mouse, 

 rat, and cat spleens. 



In 1941, MacKenzie et al. (83) reported that they 

 had been unable to confirm Knisely's findings. They 

 point out that the modern concensus favored a 

 splenic circulation that had an open component 

 which allowed flooding of the pulp interstices with 

 whole blood, and additional pathways afforded a 

 closed circulatory component. The reception of 

 Knisely's investigations had been favorable and 

 "offered a reasonable conclusion to an otherwise 

 apparently interminable discussion.'' However, they 

 were not able to see what Knisely had seen. In 

 transilluminated spleens of mice, the walls of follicle 

 arteries were seen as sharply refractile lines, running 

 parallel, the diameters of the vessels uniform except 

 when constriction occurred. The follicle arteries 

 branched two or three times to form penicilli, syno- 

 nyms being pulp arteries, sheathed arteries, or pulp 

 arterioles. They were able to see only the peripheral 

 portion of the follicle capillary network, the ultimate 

 twigs penetrating the marginal zone of the red 

 pulp. Terminal capillary branches, as many as eight 

 in number, enter the adjacent red pulp and develop 

 funnel-shaped dilatations. These arteriocapillary 

 ampullae communicate directly with the pulp 

 interstices by way of numerous apertures. As the 

 lumen of the capillary widens in the formation of its 

 ampulla, the refractive quality of the vessel wall is 

 rapidly lost. The parallel linear shadows produced 

 by the capillary are replaced by the contours of pulp 

 cells. 



Venous sinuses originate in the red pulp by an 

 enclosure of pulp spaces. A venous sinus gradually 

 increases in diameter to a maximum and then joins a 

 vein. The wall of the venous sinus is composed of 

 loosely connected cells lying parallel to the long 

 axis. The openings between these cells, according to 

 MacKenzie et al., permit the free passage of blood 

 cells. 



The interstices of the pulp provide the one and only 

 type of connection seen by them to link the arterial 

 and venous systems in the spleens of mice, rats, 

 rabbits, guinea pigs, and cats. They state, however, 

 that in all spleens there were instances when an 

 arterial capillary appeared to be connected by a 

 vessel to an adjacent venous sinus, but this proved to 

 be an optical illusion caused by weaknesses inherent 

 in the transillumination technique. Spontaneous 



arterial vasoconstrictions were seen to occur inter- 

 mittently and were a factor in the control of circula- 

 tion of blood through the small vessels as were trabec- 

 ular and capsular contractions. MacKenzie et al. 

 believed that the results of their work supplied 

 additional confirmation of an open circulation for the 

 mammalian spleen. 



Bjorkman (11) studied rabbit spleens following the 

 injection of starch granules and concluded from the 

 distribution of the grains that circulation through the 

 spleen was the open type. 



A detailed and convincing report in favor of the 

 closed system of circulation appeared in 1 951 , 

 authored by Peck & Hoerr (94). They selectively 

 attacked statements made in the paper by Mac- 

 Kenzie et al. (83), pointing out where possible 

 technical variances could explain the differences in 

 their observations. Peck and Hoerr found the inter- 

 mediate circulation of the spleen of the mouse to be 

 essentially as Knisely (6g) described it. They say that, 

 on arriving at the red pulp, arteries branch two to six 

 times to form the penicillar arteries which then 

 extend into the red pulp 10 to 15 u before branching 

 several times to form capillaries. Where more than two 

 capillaries arise from a red pulp artery, the termina- 

 tion of the artery may be ampulla-shaped (see fig. 15). 

 There is no discontinuity of the refractile lines from 

 artery to capillary, and these lines caused by the 

 vessel walls must be endothelium or reticular fibers. 

 Capillaries extend in all directions from the ampulla- 

 like terminations of the penicillar arteries to terminate 

 in venous sinuses or venules (fig. 16). 



The course taken by a capillary may be straight, 

 curved, or tortuous. If they have a tortuous course it 

 is often difficult to follow them because they run 

 under other vessels or extend beyond the range of 

 focus. Capillaries may turn away from view at their 

 point of origin, giving the ampulla-like termination 

 of the penicillar artery the appearance of ending in a 

 sac or pouring its blood into the pulp. The capillaries 

 can be seen to terminate in the venous sinuses, the 

 walls of which (more difficult to see in the contracted 

 spleen) are continuous with the walls of the capillary. 



Penicillar arteries have a powerful sphincter action, 

 although an artery may contract along its entire 

 length. Individual arteries may exhibit this con- 

 striction independently of neighboring arteries, which 

 continue to have a rapid flow. Fairly constant blood 

 flow over a period of hours is seen in straight capil- 

 laries which terminate in venules. In some cases red 

 cells may seem to wander in the extravascular tissue, 



