124 ANIMAL PHYSIOLOGY. 



appears altogether, and the middle coat consists of inusclG 

 only. 



The advantage of the elasticity of the arterial walls may 

 be easily illustrated. If a glass tube have a nozzle fastened 

 into it at one end, and at the other be fitted to the stop-cock 

 of a water pipe, and if the water be turned on and off alter- 

 nately so as to imitate the repeated discharges of blood 

 from the ventricles, the water will emerge from the nozzle 

 in jets, which will cease instantaneously each time that it is 

 turned off. But if the same experiment be made with a 

 long india-rubber tube instead of a glass one, the water will 

 spring from the nozzle in a continuous flow, notwithstanding 

 the interrupted manner in which it is admitted to the tube ; 

 and if the experiment be varied so that the glass and the 

 india-rubber tube shall both be filled from the top at the 

 game time, while the nozzles on the two tubes are of the same 

 size, the elastic one will discharge in a given time a much 

 larger quantity of water than the one which is rigid. In the 

 rigid tube there is great loss of force by friction ; while in 

 the elastic tube, as each fresh jet of fluid enters, the walls 

 are distended, and as it ceases they recover, and give their 

 contents a fresh propulsion onwards in a second wave, which 

 distends the tube further on ; and thus, after traversing a 

 sufficient length of tube, the interrupted stream is converted 

 into one which is continuous. This is precisely what happens 

 in the arteries. When a large artery is divided, the blood 

 comes in separate abrupt jets with well marked intervals 

 between ; in smaller arteries the duration of the jets is longer 

 and the intervals are shorter, and from little twigs the blood 

 spouts out in an almost continuous stream. 



While the elasticity of the arteries thus converts the sepa- 

 rate gushes of blood from the heart into one continuous flow 

 before the capillaries are reached, their contractility, derived 

 from their muscular fibres, determines the amount of blood 

 which is sent at different times to each part. H Their contrac- 

 tion is not of the vermicular description, but purely tonic : 

 they do not assist the forward movement of the blood by 

 propelling it onwards, but, by varying in diameter at different 

 times, they allow more or less blood to pass through them. 

 The muscular fibres are governed by nerves, termed vaso- 



