LOCAL VARIATIONS. 351 



DURATION OF THE CIRCULATORY MOVEMENT. 



In the Horse 28 seconds. 



" Dog 15 



" Goat 13 " 



" Rabbit 7 " 



These results are corroborated by subsequent investigations. In ex- 

 perimenting upon the dog, by injecting a solution of potassium ferrocy- 

 anide into the jugular vein, and immediately drawing blood from the 

 corresponding vein on the opposite side, we have found that the short 

 interval of time requisite for closing the first vein by ligature after 

 terminating the injection, and opening the second in such a manner as 

 to obtain a specimen of blood for examination, is sufficient to allow 

 of the passage of the ferrocyanide through the entire round of the cir- 

 culation. If we regard the duration of this movement in the human 

 subject as intermediate between that in the dog and the horse, making 

 allowance for the difference in size, this would give the time required 

 by the blood to make the circuit of the veins, arteries, and capillaries, 

 in man, as not far from 20 seconds. 



Local Variations in the Capillary Circulation. 



An important class of phenomena connected with this part of the 

 subject consists of the local variations in the capillary circulation. 

 These variations are often very marked, and show themselves in many 

 different parts of the body. The pallor or suffusion of the face under 

 mental emotion, the congestion of the mucous membranes during diges- 

 tion, and the local and denned redness of the skin produced by any 

 irritating application, are all instances of this sort. These changes are 

 due to the contraction or dilatation of the smaller arterial branches 

 which supply the part with blood, under the influence of nervous action. 

 The middle coat of these vessels is composed mainly of organic or 

 unstriped muscular fibres, arranged in a transversely circular direction, 

 which by their contraction diminish and by their relaxation enlarge 

 the calibre of the arterial tube. They regulate, accordingly, by this 

 means, the quantity of blood passing to the capillary system. When 

 contracted, they resist more strongly the impulsive force of the arterial 

 current, and admit the blood in smaller quantity. When dilated, they 

 allow a freer access to the capillaries and the blood passes in greater 

 abundance. 



These changes are most distinctly manifested in the' periodical con- 

 gestion of the glandular organs. All the glands and mucous membranes 

 connected with the digestive apparatus enter into a state of unusual 

 vascular excitement at the time of secretion and digestion. This can 

 readily be seen, in the living animal, in the pancreas, and in the mucous 

 membranes of the stomach and small intestine ; the tissues of these 

 parts being visibly redder and more turgid during digestion and absorp- 

 tion than in the fasting condition. 



A similar variation of the circulation has been particularly studied 



