MOTION OF THE BLOOD IN THE VEINS. t 497 



in the capillary vessels of the frog will continue for several minutes after the 

 interruption of the heart's action.] In a short time, the blood again began to 

 move, but with less force. This experiment was repeated, with the same result. 

 If the brain is not completely crushed, although the animal is killed, the blow, 

 instead of destroying the circulation, increases the rapidity.' 71 We are not 

 hence to conclude, however, that the Nervous system supplies any influence 

 which is essential to the continuance of the Circulation ; since it is only by such 

 sudden and severe injuries to the nervous centres, as instantaneously destroy 

 the vitality of the whole system ( 321), that the movement of the blood is 

 arrested. The experiments of Miiller and others satisfactorily prove that the 

 ordinary action of the Nerves does not produce any direct effect upon the 

 capillary circulation) and this corresponds with the well-known fact that the 

 Nutritive processes may continue as usual after this action has been suspended. 

 All the facts which bear upon the question of the connection between Nervous 

 agency and the forces maintaining the capillary circulation have an equal rela- 

 tion to the functions of Nutrition and Secretion in general; and, as already 

 shown, the Nervous system also influences these, by the control it exerts over 

 the diameter of the bloodvessels ( 515). 



529. The average rate of movement of the blood through the capillary sys- 

 tem may be determined with tolerable precision by microscopic measurement ; 

 and the observations of Hales, Valentin, and Weber concur in representing 

 it to be from 1 inch to 1? inch per minute in the systemic capillaries of the 

 Frog; 1.2 inch per minute, or .02 inch per second, being about- the average. 

 In warm-blooded animals, however, the capillary circulation is probably much 

 more rapid than this; the observations of Yolkmann upon the mesenteric arteries 

 of the Dog make its rate about .03 inch per second, or 1.8 inch per minute; and it 

 seems reasonable to suppose that the exposure of the membrane to the cool air 

 would produce a considerable reduction in the normal rapidity of the flow of 

 blood through it. Assuming .03 inch per second, however, as the rate, and 

 comparing this with the rate of movement of the blood in the larger arteries, 

 which seem on the average to be 11.8 inches per second, it is calculated by 

 Volkmann that the aggregate area of the capillaries (being in an inverse ratio 

 to the rate of the blood's movement through them) must be nearly four hundred 

 times that of the arterial trunks which supply them. 2 



5. Motion of the Blood in the Veins. 



530. The Venous system takes its origin in the small trunks that are formed 

 by the reunion of the Capillaries; and it returns the blood from these & the 

 Heart. The structure of the Veins is essentially the same with that of the 

 Arteries ; but the fibrous tissue of which their middle coat is made up, bears 

 more resemblance to the areolar tissue of the skin, than it does to the true elastic 

 tissue ; and the muscular fibre-cells are usually much fewer in number, and are 

 sometimes wanting altogether. 3 The elasticity of the Veins is shown by the jet 

 of blood which at first spouts out in ordinary venesection, when, by means of 

 the ligature, a distension has been occasioned in the tubes below it. A slight 

 contractility on the application of stimuli, and on irritation of the Sympathetic 

 nervous fibres, has been observed ; but this is not so decided as in the arteries. 



1 "Experimental Inquiry into the Laws of the Vital Functions," 4th edition, p. 52. 



2 "Hiimodynamik," pp. 184, 204. 



3 The following, according to Prof. Kolliker, are veins which are unprovided with mus- 

 cular structure : The veins of the uterine portion of the placenta ; the veins of the cere- 

 bral substance, the sinuses of the dura mater ; Breschet's veins of the bones ; the venous 

 cells of the corpora cavernosa in the male and female ; and probably the venous cells of 

 the spleen. (" Kolliker and Siebold's Zeitschrift," 1849.) 



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