158 



THE BLOOD AND ITS CIRCULATION. 



in the thorax : the positive waves then increase (see Respira- 

 tion). It has been thought that under certain circumstances 

 the right pulse is more or less rapid than the left : this is 

 what is called the differing pulse. This supposition arose 

 from errors in observation. The difference is simply due 

 to accidental rhythmical contractions of satellite muscles 



of the arteries, the cora- 

 co-brachial, for instance, 

 in the case of the radial 

 pulse. 



Besides these elastic 

 properties, belonging to 

 the muscle and to the 

 yellow tissue, by means 

 of which the arteries 

 regulate the general cir- 

 culation, these vessels 

 have also power, by the 

 contraction of their 

 smooth muscles, to 

 change their size con- 

 siderably, and in this 

 way influence the circu- 

 lation. As these mus- 

 cles abound in the small 

 vessels (see Fig. 50), it 

 is principally the local 

 circulations which are 

 thus modified, these 

 variations in diameter 

 being scarcely observ- 

 able in the large arte- 

 ries. In general, the 

 small arteries contract 

 more or less, according as 

 they may be more or less 

 well nourished. These 

 contractile properties 

 are made use of in sur- 

 gery, and the hemostat- 

 ics employed are useful, 

 not only because they coagulate the blood, but also because 



* Irregular contractions of the small vessels of the interdi^ital membrane of 

 a frog. The contraction is produced by irritation. (Whartou Jones.) 



Fig. 64. Contraction of the small arteries.* 



