CIRCULATION. 63 



Unnatural frequency of pulse depends either on func- 

 tional disturbance of the cerebro-spinal centres or increased 

 bodily temperature (pyrexia). Unnatural celerity of pulse, 

 i.e. diminished duration of the period of expansion, is ob- 

 served either in association with increased frequency, or 

 in consequence of mechanical defect of the aortic valve. 

 The opposite condition (tardiness of pulse) occurs in 

 advanced age, and in collapse (e.g. in concussion), or under 

 the influence of certain drugs, particularly of opium and 

 digitalis. 



Velocity of the Circulation. The velocity of the blood- 

 stream is dependent on the relation of the effort made by 

 the heart to the resistance to be overcome in the vessels. 

 This resistance varies according to the condition of the 

 vascular nervous system. 



It is believed that the velocity of the blood-stream in 

 the aorta is about a foot in a second ; in the capillaries 

 about i-5oth of an inch ; and that the circulation is 

 accomplished in the time occupied by about thirty pulsa- 

 tions of the heart. 



Of the instruments used for investigating the progressive 

 movement of blood in the arteries, those by which the 

 quantity conveyed in a given time can be estimated are 

 called Dromometers (e.g. Volkmann's and Ludwig's), those 

 by which the rate of movement only is determined, Tacho- 

 meters. By the latter we learn that in the large arteries 

 the second beat or expansion is attended by a cessation or 

 even reversal of the blood-stream ; by the former, that the 

 rate at which blood is transmitted through an artery is 

 subject to great and often sudden variations, and that 

 these variations are independent of the rate at which blood 

 is discharged into the arterial system by the heart. 



Capillary Circulation. In studying the circulation in the 

 transparent parts of animals, we observe that the smallest 

 arteries are subject to great variations of diameter, and 

 often contract rhythmically ; that the progressive move- 



