68 ELIOT R. CLARK 



important. For while in some capillaries there is diminution in 

 rate before retraction, others become smaller and have the lumen 

 reduced to zero through which the rate never has become slower. 

 The only common factor between these two is the total amount 

 of blood flowing through. In the second type this is shown by 

 the fact that though the flow is rapid, it is scanty and intermittent. 

 The question naturally arises why the circulation becomes 

 slower and even ceases altogether in certain capillaries, in a 

 region where an active new growth of capillaries is taking place. 

 In the case of many of the capillaries, the answer is pretty clear. 

 With the continuous formation of new vessels, the pressure con- 

 ditions in the vessels already present are changed, and a capil- 

 lary which formerly may have been the only vessel between vein 

 and artery, may later form merely a cross connection between 

 two parallel vessels, in which the pressure is equal. With an 

 equal pressure in the two ends of the capillary, the flow of blood 

 through the capillary ceases. The majority of the cases are 

 evidently of this character, as may be seen by looking over the 

 records (cf. branches 12 and 16, figs. 1 to 5). There are, how- 

 ever, other vessels to which this does not seem to apply, particu- 

 larly branches of the larger arterioles (cf. branch 3, figs. 1 to 6). 

 A considerable percentage of the branches of each vessel which 

 differentiates into an arteriole disappear. In the case of these 

 capillaries the retraction is preceded by a period during which 

 there is no flow through the vessel, but the period of absence of 

 circulation is not preceded by a slowing of the circulation. In- 

 stead of a slowing there is a diminution in the amount of blood 

 passing through, while the rate remains rapid. Periods of total 

 absence of circulation alternate with periods in which a few cells 

 pass through at a rapid rate. In this condition the capillary 

 branch affected leaves the arteriole at a right angle, and 

 is relatively small as compared with the arteriole. Frequently 

 the entrance to the branch becomes plugged by an erythrocyte 

 or a leucocyte, which causes a stoppage of the circulation. The 

 retrogression of such vessels takes place usually much later 

 than in the case of the other retrogressing vessels, such a vessel 

 often remaining for several days, with a gradual increase in the 



