ARTERIAL PRESSURE. 267 



equal to 172 millimetres in the carotid, and 165 mm. in the 

 metatarsal. In an experiment on a calf, the pressure was 116 

 mm. in the carotid, and 89 mm. in the metatarsal ; and in a 

 rabbit, 91 mm. in the carotid, and 86 mm. in the crural. 1 



These experiments, which seem to have been performed 

 with great care, show that the pressure is not absolutely the 

 same in all parts of the arterial system ; that it is greatest in 

 the arteries nearest the heart, and gradually diminishes as 

 we near the capillaries. The difference is very slight, almost 

 inappreciable, until we come to vessels of very small size ; 

 but here the pressure is directly influenced by the discharge 

 of blood into the capillaries. 



The cause of this diminution of pressure in the smallest 

 vessels is the proximity of the great outlet of the arteries, the 

 capillary system ; for, as we shall see further on, the flow into 

 the capillaries has a constant tendency to diminish the press- 

 lire in the arteries. It is obvious that this influence can only 

 be felt in a very marked degree in the vessels of smallest size. 3 



Influence of Respiration. It is easy to see, in studying 

 the arterial pressure with any of the instruments we have 

 described, that there is a marked increase with expiration, 

 and a diminution with inspiration. The fact that expiration 

 will increase the force of the jet of blood from a divided 

 artery has long been observed, and accords perfectly with the 

 above statement. 



oiily, diminishing in the smaller vessels. The instrument by no means possesses 

 the delicacy of the apparatus used by Volkmann, hi giving the mean pressure. 

 (Liquidcs de V Organisme, tomei., p. 209 et seq.) 



1 MILNE-EDWARDS, op. cit., tome iv., p. 234. 



2 This view is fully sustained by physical laws. If fluid be discharged from a 

 reservoir by a long horizontal tube of uniform caliber, the pressure, as indicated 

 by vertical tubes at different points, will be found to diminish regularly from the 

 height of the fluid in the reservoir to the orifice of discharge. An instrument of 

 this kind, which is called a piezometer, shows the apparent physical necessity 

 of a progressive diminution in pressure in the arterial system, as we pass from the 

 heart to the capillaries. 



