670 SPECIAL PHYSIOLOGY. 



tested by the sphygmographs, is simultaneous, the nearer wave reaches 

 its highest point, before the others, which reach theirs at progressively 

 later times. This is believed by Marey to happen in the living body, 

 and to explain the apparent retardation of the pulse movement, or dis- 

 tension effect, which is indicated by the pulse itself being felt Jth of a 

 second later in the wrist than close to the heart, although, from the 

 practical incompressibility of the blood, the shock imparted to it by the 

 left ventricle must be instantly propagated through the whole of that 

 fluid, in the arteries, just as that of the india-rubber cylinder is through 

 the equally incompressible water. 



The phenomenon known as the dichrotom pulse, is also detected and 

 studied by the sphygmograph. Formerly, it was supposed to be abso- 

 lutely the result of disease, or of some grave irregularity ; but with 

 more delicate instruments, its presence is often detected even in 

 healthy conditions. It is represented by a slight secondary undula- 

 tion in the down-stroke of the chief or primary pulse-line. It some- 

 times occurs, in health, during walking, and is noticeable also in the 

 healthy pulse, whenever, owing to the diminished tonicity of the arte- 

 ries, and their defective distension, they are in a condition to obey 

 slighter impulses communicated to the blood. In the opposite condi- 

 tions of a highly tonic or distended state of the arteries, this subsi- 

 diary wave motion of the dichrotous pulse, is not perceptible. In 

 abnormal conditions, it is a sign of a relaxed state of the arterial sys- 

 tem, or of a loss of blood. According to Naumann, the cause of this 

 dichrotism is the shock communicated to the blood, at the instant of 

 closure of the aortic semilunar valves, which, like the sudden arrest of 

 a fluid by the closure of a tap, produces a shock or jar, which is trans- 

 mitted back through the whole column. The time of occurrence of the 

 dichrotous pulse corresponds well with this hypothesis ; for whilst the 

 primary pulse movement is felt before the second sound of the heart, 

 the dichrotous wave immediately follows it. It is, however, suggested 

 by Marey, that this dichrotism may be due to the primary wave being 

 checked at the lower end of the abdominal aorta, where that vessel 

 divides into the common iliacs, owing to the fact, elsewhere referred to, 

 that the two iliac arteries are smaller than the aorta from which they 

 proceed, contrary to the general rule, that the area of the branches 

 exceeds that of the parent trunk. At this point, the primary pulse- 

 wave is supposed by Marey to rebound, and to produce a back wave, 

 which causes the dichrotous pulsation. In support of this explanation, 

 it is alleged, that whilst the pulse is dichrotous in all the arteries aris- 

 ing from the arch of the aorta, it is not so in the femoral arteries and 

 arteries of the lower limbs, along which the primary pulse-wave only 

 travels. But Naumann asserts that the dichrotous pulse-wave dimin- 

 ishes in force, as it recedes from the heart a fact which would sup- 

 port his view, but be opposed to Marey 's ; for, in the former case, the 

 wave is supposed to travel outwards from the heart, but in the latter, 

 towards the heart, i. e., from the lower end of the abdominal aorta. 

 Another suggestion has been made, viz., that whilst a primary wave 

 occurs in the blood, a secondary wave follows it in the coats of the ves- 

 sels ; this opinion rests upon an experiment, in which it was shown 



