MOVEMENT OF THE BLOOD IN THE VEINS. 353 



of the smaller arteries, which \vill thus regulate the rate at which the blood 

 is supplied to the Capillary plexus ; and, second, the direct agency of Nerve- 

 force in stimulating, retarding, or modifying those molecular changes, in 

 which ihe Nutritive and Secretory operations consist. (See I'KIN. OF GEN. 

 PIIYS.) That the ordinary action of this force is not required to sustain the 

 Capillary circulation, is clearly proved by the continuance of the flow with- 

 out any apparent alteration, after section of the nerves of the part, as has 

 been observed by Midler, Whartou Jones, and others ; and this corresponds 

 with the well-known fact, that the Nutritive and Secretory processes may 

 take place after Nervous agency has been thus suspended. But it seems 

 indubitable that a sudden and violent "shock" to the nervous centres may 

 exert the same antagonistic influence on the movement of blood in the Cap- 

 illaries, as we have seen it to do on the Heart's action ( 244) : for this 

 appears alike from the immediate and total annihilation of all vital activity 

 which is consequent upon such an injury, and from direct observation in such 

 an experiment as the following, made by Dr. Wilson Philip. "The web of 

 one of the hind legs of a frog was brought before the microscope ; and while 

 Dr. Hastings observed the circulation, which was vigorous, the brain was 

 crushed by the blow of a hammer. The vessels of the web instantly lost their 

 power, the circulation ceasing ; an effect which cannot arise, as we have seen, 

 from the ceasing of the action of the heart. [Dr. Philip here refers to experi- 

 ments, by which it was ascertained that the circulation in the capillary ves- 

 sels of the frog will continue for several minutes after 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 its rapidity." 1 



5. Movement of the Blood in the Veins. 



273. 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 

 to 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 presents the distinguishing feature of Laminae run- 

 ning longitudinally as well as transversely; and the muscular fibre-cells are 

 usually much fewer in number, and are sometimes wanting altogether. 2 

 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. Though the walls of the Veins are 

 thinner, they yet resist pressure better than the Arteries. In an experiment 

 of Wintringham 3 the Aorta burst when the pressure rose to 158 Ibs., whilst 

 the Vena Cava only gave way when it reached 176 Ibs. A slight contrac- 

 tiliti/ 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. 

 The whole capacity of the Venous system is considerably greater than that 



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



2 The following, according to Prof. Kolliker (Manual of Human Histology, Svd. 

 Soc., vol. ii, p. 307), are Veins which are unprovided with muscular structure, : Thu 

 veins of the uterine portion of the placenta ; the veins of the cerebral substance and 

 pia mater ; the sinuses of the dura mater; Breschct'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. See also Eherth, art. Bloodvessels, in Strieker's Hum. and (Jomp. 

 Histology, vol. i, 1870, p. ^75. 



3 Experimental Inquiry, etc., 1740. 



