324 OF THE CIRCULATION OF THE BLOOD. 



rial walls, which is a physical property that persists after death, until a seri- 

 ous change takes place in their composition ; whilst to the latter we are to 

 attribute the property which they unquestionably possess (in common with 

 proper muscular tissue), of contracting on the application of a stimulus, so 

 long as their vitality remains. These two endowments are possessed in vari- 

 ous degrees, proportional to the respective predominance of the elastic or of 

 the muscular tissue, by the different parts of the Arterial system. Thus, as 

 was justly remarked by Hunter, the elasticity, being the property by which 

 the interrupted force of the Heart is made equable and continuous, is most 

 seen in the large vessels more immediately connected with that organ ; whilst 

 on the other hand, the contractility is most observable in the smaller vessels, 

 where it is more required for regulating the flow of blood towards particular 

 organs. With the exception of the capillaries, nerves partly consisting of 

 dark-edged and partly of pale fibres have been demonstrated in all vessels, 

 even in the tunica adventitia of the non-muscular veins of the pia mater. 

 The fibres, 1 after they have traversed the tunica adveutitia, break up into a 

 fine network with here and there ganglionic cells and knots. It is to these 

 ganglionic enlargements that the local contractions of the vessels are due, 

 which take place when the main trunks from the sympathetics which supply 

 them have been divided. They appear to be easily excitable, but soon ex- 

 hausted. 2 



251. The chief purpose served by the Elasticity of the Arteries, is one of 

 a purely physical character ; its effect being to convert the intermitting im- 

 pulses, which the blood receives from the heart, into a continuous current. 

 The former are very evident in the larger trunks; but they diminish with 

 the subdivision of these, until they entirely disappear in the capillaries, in 

 which the stream is usually equable or nearly so. If a powerful force-pump 

 were made to inject water, by successive strokes, into a system of tubes with 

 unyielding walls, the flow of fluid at the farther extremities of these tubes 

 would be as much interrupted as its entrance into them. But if the air- 

 vessel (like that of a fire-engine) were placed at their commencement, the flow 

 would be in a great degree equalized ; since a part of the force of each stroke 

 would be spent upon the compression of the air included in it ; and this force 

 would be restored by the elasticity of the air during the interval, which would 

 propel the stream, until directly renewed by the next impulse. A much closer 

 imitation of the natural apparatus would be afforded by a pipe which had 

 elastic walls of its own ; thus if water were forced by a syringe into a long 

 tube of caoutchouc, for example, the stream would be equalized before it had 

 proceeded far. This effect is found' to be accomplished, at any point of the 

 Arterial circulation, in a degree proportionate to its distance from the Heart ; 

 and in this mode it is, that the intermitting force of the ventricular contrac- 

 tion is almost equably distributed over the whole of the interval between one 

 systole and another, by the contraction of the elastic tubes in the dilatation 

 of which it was at first expended. Another effect of this elasticity is to dis- 

 tribute the pressure of the blood upon the walls of the arteries much more 

 equally than would be the case if they formed a system of rigid tubes. 



252. The contractility of the arteries fulfils several important purposes. It 

 is the principal means by which hternorrhage from the smaller vessels is ar- 

 rested after injury, and even in the case of so large an artery as the temporal, 



ticity is greater in the longitudinal than in the transverse direction. The coefficient 

 of elasticity of the aorta, taking the mean of several experiments, is 927 grains in 

 the direction of its length and 588 grains in the direction of its hrcadth, whilst that 

 of the jugular vein is 150:'. grains in the former and 72") grains in the latter direction. 



1 Eberth, in Strieker's Hum. and Compar. Histol., Syd. Soc. Transl., vol. i, p. 26G. 



2 See Mohammed Eft'cndi Hafiz, in Ludwig's Arbciten, 1871, Bd. v, p. 95. 



