CAPILLARY COUNTS IN RESTING AND ACTIVE MUSCLES

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Abstract
In dogs the intact gracilis muscle (in some cases resting, in others stimulated by brief tetanic shock) was flooded with India ink under the prevailing arterial pressure, the muscle excised, fixed, sectioned (stained or not). The fasciculi appear to be the circulatory units in the sense that in the "rotation of function" among the capillaries, typical of skeletal muscles, the capillaries of individual fasciculi tend either to be closed en masse or to be open (receptive of India ink upon mass injection of ink into the muscular vessels) to an extent characteristic of muscles in a state of rest When so open the ratio of number of open capillaries: number of muscle fibers averages about 0.64. During exercise, toward the end of long and drastic experiments on the entire animal, and both early and late after stoppage of the circulation, the number of open capillaries averages about double the number in injected fasciculi of resting muscles. A few unin-jected fasciculi are seen in muscles injected after 4-5 min. of exercise, none in muscles under the other conditions described. The marked agreement among the capillary counts under the diverse conditions cited is interpreted to mean a general relaxation of all the capillaries to a "resting" state, which is explained by capillary constrictive influences rather than by the onset of positive capillo-dilator influences. After administration of amyl nitrite the ratio of number of open capillaries:number of muscle fibers is 2-2.5, as compared with about 1.2 for the corresponding ratio under the conditions first cited. An old assumption may explain this marked increase in number of open capillaries, i.e., the presence, in addition to those capillaries that can be injected under the conditions first noted, of many additional capillaries, too small in caliber to be injected even when normally relaxed, but capable of relaxing further under the influence of powerful capillo-dilators to a caliber which will take ink. In terms of no. of units per sq. mm. of cross section the following are the av. data for gracilis muscles: muscle fibers, 1640; open capillaries after amyl nitrite, 2580; in injected areas of resting muscle, 1050; during muscular exercise, 2010; in the late stages of long experiments, 1820; 1/2 hr. after replacing the blood with Ringer''s fluid, 2185; 24 hrs. after excision of the muscle, 1790.

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