354 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



of blood into tlie arterial system from the heart. The walls of the 

 arteries have nothing to do with the causation of the pulse, but 

 may render it more or less distinct,, according as they are more or 

 less yielding. 



The character or quality of the pulse will depend primarily and 

 essentially upon the force of the heart, secondly, upon the integrity 

 of the mechanism by which that force is directed, so as to drive the 

 blood into the arteries, thirdly upon the quantity of blood in the 

 vascular system, and, fourthly, upon the condition of the arterial 

 walls, according as it is apt to oppose or to yield before the 

 wave caused by the heart's action. By reference to these points, 

 we may explain the various conditions of the pulse observed in 

 practice. Thus, a weak heart, or a contracted arterial aperture, or 

 a small supply of blood, will each equally produce a small pulse ; 

 while a certain power of heart, and an open unimpeded state of the 

 arterial aperture, with a full supply of blood, are quite necessary to 

 the formation of a large round pulse. But the qualities of soft- 

 ness or fulness, of hardness or wiryness, of compressibility, of 

 incompressibility, all which are familiar to the tactus expertus of 

 the practical man, are determined by the yielding or the resisting 

 condition of the arterial wall. 



Contractility of Arteries. Arteries possess a power of contraction 

 in virtue of the large quantity of elastic material which enters into 

 the constitution of their wall; but this is a contraction which 

 may occur in a dead as well as in a living artery, and which simply 

 serves to restore to its medium dimensions, an artery previously dis- 

 tended or stretched. They have, however, also a power of active 

 contractility, which ceases with life, which is capable of being called 

 into play not only by distension, but by other appropriate stimuli, 

 and which can diminish the size of the vessels far beyond what their 

 mere elasticity could effect, and even against the influence of the 

 elastic force. This contractile power is due to the presence of 

 unstriped muscular fibres in the arterial wall. The demonstration 

 of these fibres in the walls of arteries by the microscope, leaves no 

 more doubt of the existence of a muscular contractile force in them, 

 than of its existence in the oesophagus or the intestine. Experiment 

 anticipated anatomical research in pointing out that arteries con- 

 tract as tubes do whose walls contain muscle, and it also indicated 

 the peculiar manner in which the muscular fibres of arteries act. 

 Under the influence of a stimulus, even of so light a nature as 

 exposure to the air, an artery may be observed to contract very 

 gradually, and to become very much diminished in size. Thus, in 



