MOVEMENT OF T1IE BLOOD IN THE ARTERIES. 329 



the tone of the vessels. There are certain nerves the irritation of which 

 causes dilatation of the vessels to which they are distributed. This is the 

 case with the chorda tympani and with the nervi erigentes of the penis 

 (Loven ), and there is considerable difficulty in explaining their action. 

 The most reasonable view is that of Vulpian, who supposes that there are 

 inhibitory centres in the walls of the vessels, which, when irritated, restrain 

 or abolish the tonic influence of the vaso-motor centre, and dilatation is the 

 immediate consequence. 1 The mode in which variations of pressure affect 

 the pulse has already been explained, 249. 



255. The distension of the Arteries that is consequent upon the intermit- 

 tent injection of blood into their trunks gives rise to the pulsation, which is 

 perceptible to the touch in all but the smallest arteries, and which is visible 

 to the eye when they are exposed. It would seem to the touch, as if this 

 were chiefly effected by an increase of diameter, and the experiments of 

 Volkmauu (Hamodynamifc) led him to believe that the transverse dila- 

 tation is greater than the longitudinal ; but these experiments were made 

 under conditions so different from those of the living artery, that but little 

 weight can be attached to them. It is to be remembered, however, that 

 every increase in length augments the capacity in only a simple ratio; thus 

 a tube of 21 inches in length will only contain one-twentieth more than a 

 tube of 20 inches long, of the same diameter. On the other hand, every in- 

 crease in diameter augments the capacity of the tube in the ratio of the 

 square of that increase; thus the capacity of a tube of 21 lines in diameter 

 will be to that of a tube of 20 lines, as 441 : 400, or one-tenth more. Conse- 

 quently, supposing the increase of capacity to take place equally in both di- 

 rections, the increase in longitudinal dimension will be far more apparent 

 than the transverse enlargement. The total increase of capacity was esti- 

 mated by Flourens, from experiments upon the carotid artery, at about ^Vd 

 part ; but it is affirmed by Volkrnann 2 that this must not be considered by 

 any means a constant ratio, since it varies in different arteries, and in the 

 same artery under different circumstances. It has also been attributed to 

 elongation of the artery, which was supposed to be lifted from its bed at each 

 pulsation, and, when previously straight, to become curved; the impression 

 made upon the finger by such displacement, not being distinguishable from 

 that which would result from the dilatation of the tube in diameter.- A very 

 obvious example of this upheaval is seen in the prominent temporal artery 

 of an old person. The real cause of the pulse, however, may be considered 

 to be the return of the artery to a cylindrical form under increased internal 

 pressure, after it has been more or less flattened by the pressure of the finger 

 against an unyielding structure, while in a comparatively flaccid condition. 

 The fluctuations of the Pulse in the living body without exposure of the ar- 

 tery can be rendered apparent by the use of an instrument originally in- 

 vented by Vierordt, but materially modified and improved by Marey, termed 

 the Sphygmograph. The general arrangement of the instrument is shown 

 in Fig. 130, where the part B B is applied to the forearm, whilst the pres- 

 sure on the artery is effected by a very light steel spring, terminating in a 



1 Vulpian, Lemons sur 1'Appari'il Vaso-moteur, 1875, p. 177etseq. Other views 

 that have ln-en advanced are: 1. That of Sell iff', who believes it to be possible that 

 certain nerves may, instead of causing contraction, cause elongation of muscular 

 fibres. '2. That which attributes the dilatation of the arteries to increased blood- 

 pressure caused by contraction of the veins; but this is contrary to fact, the veins 

 being dilated in all cases. 3. That of M. Legros (These, 1873), who regards active 

 congestion as the result of increased peristaltic action of the vessels. 4. That it is 

 due to increased attraction of the tissues for tlie blood, so that a suction force or uis 

 a frotife is established. 



2 Up. cit., chap. xiv. 



22 



