362 THE CIRCULATION OF THE BLOOD. [CHAP. xxvm. 



hydrodynamic fact, as also with the results of other observations, 

 of which those of Volkmann seem the most trustworthy. Volkmann 

 shows that a fluid flowing through a system of tubes has to 

 encounter at the point of its entrance into it the sum of the resist- 

 ance which opposes it throughout the entire area; and these resist- 

 ances determine the amount of pressure needed for its propulsion. 

 Applying this to the arteries, it is plain that in them the blood has 

 to encounter the resistances in the capillaries, in the small arteries, 

 in the middle-sized arteries, and in the arterial trunks ; a fact, 

 which by assigning to the resistance in each of these regions the 

 symbols x, y, z, w t x representing that in the arterial trunks, and 

 the other letters that in each of the remaining segments of the 

 system, may be thus expressed, P (being the pressure in the 

 commencement of the arterial t?ee)=x+y -\-z-\-iu ; whence it is 

 plain that the amount of pressure cannot be the same in all parts 

 of the arterial system, but diminishes steadily as the artery is more 

 distant from the heart. 



Spengler' s experiments so far disprove the accuracy of Poiseuille's 

 statement, as to show that the pressure of the blood differs consi- 

 derably in different arteries ; but in the greater number of his 

 observations, the pressure appeared to be greater in the arteries 

 more distant from the heart, some showing a difference of 36 mil- 

 limetres in favour of the more distant artery ; but in one of the 

 instances quoted by Volkmann from Spengler, there was a difference 

 of 16'6 millimetres in favour of the carotid artery as compared 

 with the maxillary. 



But Volkmann' s very numerous observations, made with more 

 perfect instruments than those used by Spengler, show a marked 

 difference of pressure in the near, and in the distant arteries. Thus, 

 eight observations on the carotid, and a branch of the femoral artery, 

 of a large dog, gave a mean of 7*2 millimetres (0'27 inch) in 

 favour of the carotid. And ten observations on the carotid and 

 metatarsal arteries of a calf yielded a mean of 27 millimetres (1/05 

 inch) in favour of the carotid; and twelve observations on the 

 same arteries of another calf, gave a mean of 19*5 millimetres also 

 in favour of the carotid. Volkmann has also shown, that in the 

 same artery a notably greater pressure exists in the part near the 

 heart, than in that more remote from it ; which is, of course, the 

 more conspicuous, as the distance between the two points measured 

 is greater. 



Influence of Systole and Diastole. Hales had observed, in his 

 experiments with a simple glass tube, that a rise and fall took place 



