PATTERNS OF THE A-V PATHWAYS 



9'3 



of several red cells abreast. The collecting veins are 

 described as thin-walled structures randomly per- 

 forated with holes of varying sizes that are part of a 

 branched-treelike arrangement. Although capillary 

 networks supplied by an arteriole and feeding into 

 veins are occasionally seen, there are relatively few 

 of them. The capillaries are said to have holes in their 

 walls through which blood enters or leaves the ad- 

 jacent pulp space. 



Parpart et al. report that they have never seen a 

 venous sinus of the type described by Knisely (69), 

 nor have they seen any activity in the venous pulp 

 spaces that could be regulatory to the blood flowing 

 through them. This is in direct opposition to state- 

 ments by Knisely (71) and by Peck & Hoerr (94) 

 regarding the regulation of blood flow through the 

 splenic pulp. Knisely (71) took exception to the 

 conclusions of Parpart et al. (93), particularly pointing 

 out that their optical arrangements were such that 

 not all structures present in the tissue would neces- 

 sarily be observed. With the quartz rod, which 

 Knisely used, it is possible to direct the light first one 

 way and then another and thereby make previously 

 unobserved structures visible (69). 



In 1958, Snook (115), who believed one reason for 

 disagreement concerning splenic circulation was the 

 structural variability of the spleen among mammals, 

 reported on fixed rabbit spleens. He had previously 

 classified the mouse with mammals that had non- 

 sinusal spleens (114), and comparative studies 

 showed that the rabbit spleen was more nearly like 

 human spleen than the other animals observed. 

 Conclusions from his histological studies of the rabbit 

 were that rabbit spleen had the open type of inter- 

 mediate circulation, that white pulp capillaries 

 occasionally connected directly with premarginal 

 sinuses, and that penicillar branches terminated in 

 pulp cords in ampullary dilatations. 



In the 1958 edition of Bailey's Textbook of Histology 

 (112) the authors take the stand that "there is a 

 fairly direct connection from the capillary to the 

 venous sinus in most cases, but the system is open in 

 the sense that the lining membrane changes from 

 endothelial cells to flattened reticular cells, and 

 contains perforations through which erythrocytes 

 may readily pass." 



Fleming & Parpart (41) investigated the spleens of 

 young rats and found them to be very different from 

 mice. Capillary networks were seen which had a 

 pattern very similar to that of mesenteric circulation 

 (144). No venous sinus or pulp spaces were found. 



Vascular walls were easily seen and very few red blood 

 cells were free in the intercellular space. 



Fleming and Parpart suggest that such a capillary- 

 pattern in the rat is an infantile characteristic and 

 that pulp spaces develop when the animal becomes 

 more mature. They believe that the fact that endo- 

 thelial walls can be seen with such clarity in this 

 preparation proves that they could also be seen, if 

 present, in the spleens of mice. Thus the position 

 taken by Parpart for an open system of intermediate 

 circulation in the mouse spleen appears strengthened. 



It is very difficult for an unbiased reader to decide 

 in favor of one or the other types of circulation in the 

 spleen because of the convincing arguments presented 

 by the proponents of each. It has been suggested, 

 however, that the burden of proof rests upon those 

 who favor the open type of circulation, since endo- 

 thelium is universally present in every other vascular 

 system (26). 



There are one or two structural arrangements 

 described by Parpart et al. which would be unique if 

 they do exist, i.e., capillary vessels with holes in their 

 walls through which blood enters or leaves the ad- 

 jacent pulp area, and veins which have end and 

 lateral openings varying in size from 5 to 20 jx, the 

 latter openings being randomly spaced along the 

 endothelial lining of the veins. 



Williams (139) expressed the opinion that the 

 entire spleen might be thought of as a modified blood 

 vessel with certain special structures in its lumen and, 

 therefore, that the endothelial lining of the internal 

 blood channels might have a different significance 

 than elsewhere. 



Microcirculation in the Lung 



Microscopic observations of pulmonary circulation 

 date back to Malpighi (84) in 1661. Occasional 

 reports appeared in the literature from time to time 

 after this, possibly the greatest concentration being 

 in the 1930's. 



In 1930, Olkon & Joannides (gi, 92) studied the 

 pulmonary circulation in dogs, frogs, and alligators ! 

 The optical magnification was quite low by present 

 day standards (60 X) and the fact that the animals 

 were on artificial respiration and their lungs in con- 

 stant motion must have added considerable difficulty 

 to their investigation. They describe what appeared 

 to be a large capillary lying between the walls of the 

 alveoli from which many smaller capillaries were 

 given off. They believed that the single large capillary 

 surrounding the alveolus was most likely a capillary 



