CH. XXI.] 



BLOOD-PRESSURE. 



28l 



is communicated to the penis by the pudic nerves, which ramify 

 in its vascular tissue ; and after their division the penis is no 

 longer capable of erection. 



Erection is not complete, nor maintained for any time except 

 when, together with the influx of blood, the muscles mentioned 

 contract, and by compressing the veins, stop the efflux of blood 

 or prevent it from being as great as the influx. 



The circulation in the Lungs, Liver, Spleen and Kidneys will be 

 described in our study of those organs. 



Blood-pressure. 



The circulation of the blood depends on the existence of 

 different degrees of pressure in different parts of the circulatory 

 system ; there is a diminution of pressure from the heart onwards 

 through arteries, capillaries, and veins, back to the heart again. 



BP 



LV 



RA 



Fig. 270. Height of blood-pressure (BP) in LV, left ventricle. A, arteries ; c, capillaries ; 

 v, veins ; RA, right auricle ; oo, line of no pressure. (After Starling.) 



Fig. 270 represents roughly the fall of pressure along the 

 systemic vascular system. 



It falls slowly in the great arteries ; at the end of the arterial 

 system it falls suddenly and extensively just beyond the resistance 

 of the arterioles ; it again falls gradually through the capillaries 

 and veins till in the large veins near the heart it is negative. 

 Such a diagram of blood-pressure is thus very different from one 

 of velocity ; the velocity like the pressure falls from the arteries 

 to the capillaries, but unlike it, rises again in the veins. 



\\ e must now study the methods by which blood-pressure is 

 measured and recorded, and the main causes that produce varia- 

 tions in its amount. 



In order to do this in the simplest way, it will be first neces- 

 ^iry to inquire how we may measure pressure in an artificial 

 schema of the circulation. 



Take the simplest possible case of a fluid flowing from a 



