66 A Manual of Veterinary Physiology. 



pressure of the coats of the vessel on the blood. The term 

 ' tension ' is one in constant use in pathology ; pulses of high 

 and low tension are frequent accompaniments of disease, 

 and methods of measuring them have been known for some 

 time past. The simplest description of the apparatus for 

 registering the movements of a vessel is that it is a delicate 

 lever placed on the artery, the excursions of which are 

 registered on a piece of paper which travels past the end 

 of the lever. Such a tracing is represented in Fig. 3, from 

 the human carotid artery. 



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Fig. 3. 



The up-stroke x A, in the second tracing, is due to the contraction of 

 the ventricle ; from A to x, in the first tracing, the blood is flowing 

 from the peripheral arteries to the capillaries ; the descent of 

 the lever is here broken by several irregularities. The first one, 

 p, in the second tracing, is termed the first tidal or predicrotic 

 notch ; the second notch in front of C is the aortic notch ; the 

 elevation, C, is the dicrotic or recoil wave, and is due to the 

 closure of the aortic valves, and the elevation, Z>, is the post- 

 dicrotic wave. The curve above is that of a tuning fork with ten 

 double vibrations in a second (Foster, after Moens). 



With the exception of the aortic notch, physiologists are 

 not agreed as to the cause of the other irregularities in the 

 down-stroke. The dicrotic wave is often of considerable 

 practical importance, giving a dcuble beat to the pulse on 

 each single contraction of the heart. 



The instrument which records these movements, and is 

 known as the sphygmograph,does not register blood pressure, 

 it simply registers the expansion and collapse of the tube at 

 the moment the wave is passing along the vessels ; it is not 

 a representation of the wave itself. In examining a pulse- 



