240 PHYSIOLOGY CHAP. 



entrance of the blood into the arteries. In this way a portion of 



the mechanical work of the heart, which would otherwise be lost 



in overcoming the great resistance which the blood would meet in 



j making its way into the arteries, if these were rigid tubes, is saved 



i and utilised. By the same experimental schema Marey was easily 

 able to demonstrate that the amount of fluid passing through an 



> elastic tube is considerably greater than that flowing in the same 

 time through a rigid pipe, when fluid is driven through both 

 intermittently. 



Another invariable result of the intermittent character of the 

 driving force exerted by the heart on the arterial blood is the 

 production of a positive wave at each systole, i.e. a dilatation which 

 is rapidly propagated in a diminishing degree from the larger to 

 the smaller arteries, and usually dies out at the threshold of the 

 capillary network. This positive wave gives rise to the arterial 



" pulse, and is accordingly known as the sphygmic or pulse wave. 

 The rapid transmission of the pulse wave through the arteries 

 coincides with a momentary rise of blood pressure perceptible 

 to the touch, and a momentary acceleration of the blood-flow, 

 which, as we have seen, can be directly observed under the 

 microscope. 



E. H. Weber (1850) was the first to make a thorough experi- 

 mental study of the laws of wave movement. For our purpose it 

 will be sufficient to consider the fundamental principles on which 

 the complex and delicate mechanism of the production of this wave 

 depend. At each outflow of blood from the ventricles, the walls 

 of the first section of the arteries expand in consequence of the 

 sudden impact, and then by elastic reaction produce the dilatation 

 of the succeeding sections by exerting pressure on the blood with 

 which they are filled. This elastic transmission of the wave is 



, repeated in the next section, and so on. Thus, it is the blood 

 expelled from the heart which causes the wave-like dilatation of 

 the vessel walls, while the elastic reaction of the walls consequent 



1 on this dilatation propagates the wave. 



The gradual diminution of the wave in its course through the 

 arteries until it disappears at the capillary threshold is an effect 

 of the growing resistance which it encounters at each ramification 

 of the vessels. The amplitude of the wave decreases by the same 

 laws as the average pressure in the arteries. 



The velocity of transmission of the wave depends on the specific 

 gravity of the fluid, on the diameter of the vessels, on the thickness 

 of the vessel walls, and on its elastic coefficient. According to 

 Moens, it is inversely proportional to the square root of the specific 

 gravity of the fluid and to the internal diameter of the vessel, and 

 directly proportional to the square root of the thickness of the 

 walls, and their coefficient of elasticity. 



By means of the graphic method it is possible to study every 



