526 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



of the vessels must, consequently, produce pressure on the walls of the 

 blood-vessels to a sufficient degree to prevent the regaining by tM elastic 

 tissue of its position of equilibrium. Such a tension as already described 

 is one of the important factors in blood pressure. By blood pressure is 

 meant the pressure which the blood exerts on the walls of the vessels. 

 By arterial tension is meant the pressure which the walls of the vessels 

 exert on their contents. It is thus seen that these two terms are mutually 

 convertible. The pressure which the blood exerts on the walls of the 

 vessels is, of course, dependent upon the energy of the contraction of the 

 heart and the resistance which the capillaries offer to the onward motion 

 of the blood. The pressure which the elastic walls of the arteries exert 

 on their contents will, of course, again depend upon the amount of blood 

 contained in the arteries and their consequent distention, and while this, 

 again, as we shall see, is capable of being modified by different causes, it 

 is mainty dependent upon the activity of the heart. Blood pressure is 

 measured by estimating the pressure which the blood exerts on the vas- 

 cular walls. 



It has been mentioned that when an opening is made in the walls of an 

 artery the blood escapes therefrom in jets,, and it is found that the larger 

 the artery, and, consequently, the nearer the opening is to the heart, the 

 higher will be the jet of blood and the more intermittent will be the 

 flow ; this indicates that the blood pressure is, therefore, greater in the 

 large vessels than in the small arterioles, and is only what is to be 

 expected from the conditions which are necessary for the maintenance of 

 the circulation. It has been stated that the arteries subdivided into 

 smaller and smaller vessels', and, consequently, the friction proportionally 

 increases with the minuteness of the vessel. It is, therefore, evident, 

 further, that the pressure in the large arteries must be higher than in the 

 arterioles, and in all arteries higher than in the veins. 



The hlood pressure may be directly measured in any accessible artery by 

 directly connecting a manometer with the interior of the vessel (Fig. 218). Such 

 an instrument, in its simplest form, consists of a U -shaped tube containing mercury * 

 in its lower part, the distal end being free to the atmosphere, and the proximal 

 end connected directly with the interior of the blood-vessel. If the pressure in 

 the blood-vessel is greater than the atmospheric pressure, it is evident that the 

 mercury will be depressed in the proximal arm and rise in the distal arm, until 

 the difference in height between the columns of mercury in the two arms equals 

 the pressure exerted by the fluid. Such an experiment is termed a blood-pressure 

 experiment, and is readily performed on any of our domestic animals. 



To make a blood-pressure experiment, the animal should be securely 

 fastened and the artery exposed through an incision. In the dog, in which such 

 experiments may be most conveniently performed, the arteries usually experi- 

 mented on are the carotid or the femoral. To expose the carotid artery, the' hair 

 is removed from the front part of the neck, and an incision, about two inches in 

 length, made in the middle of the neck, at the anterior border of the sterno-mas- 

 toid muscle : the platysma and subcutaneous fascia are then broken through with 

 forceps or blunt hooks, and the sterno-mastoid muscle pushed to the outside, and 

 the artery is readily found lying beneath it, the pneumogastric nerve running in 

 the same sheath. The bundle containing the pneumogastric, the sympathetic, and 



