PRESSURE OF BLOOD IN THE ARTERIES. 75 



muscular coats of some of the arteries, causing them to contract and thereby 

 diminishing the total capacity of the arterial system. 



Effects of Hemorrhage etc. Diminution in the quantity of blood has a 

 remarkable effect upon the arterial pressure. If, in connecting the instru- 

 ment with the arteries, even one or two jets of blood be allowed to escape, the 

 pressure will be found diminished perhaps one-half or even more. It is 

 hardly necessary to discuss the mechanism of the effect of the loss of blood 

 on the tension of the vessels, but it is remarkable how soon the pressure in 

 the arteries regains its normal standard after it has been lowered by haemor- 

 rhage. As the pressure depends largely upon the quantity of blood, as soon 

 as the vessels absorb the serosities in sufficient quantity to repair the loss, 

 the pressure is increased. This takes place in a very short time, if the loss 

 of blood be not too great. 



Experiments on the arterial pressure, with the cardiometer, have verified 

 the fact stated in treating of the form of the pulse ; namely, that the pressure 

 in the vessels bears an inverse ratio to the distention produced by the con- 

 tractions of the heart. In the cardiometer, the mean height of the mercury 

 indicates the constant, or arterial pressure ; and the oscillations, the disten- 

 tion produced by the heart. It is found that when the pressure is great, the 

 extent of oscillation is small, and vice versa. It will be remembered that 

 the researches of Marey demonstrated that an increase of the arterial 

 pressure diminishes the amplitude of the pulsations, as indicated by the 

 sphygmograph, and that the amplitude is very great when the pressure is 

 slight. It is also true, as a general rule, that the force of the heart, as in- 

 dicated by the cardiometer, bears an inverse ratio to the frequency of its pul- 

 sations. 



Depressor Nerve of the Circulation. Cyon and Ludwig have described 

 a nerve arising in the rabbit, by two roots, one from the main trunk of the 

 pneumogastric and the other from the superior laryngeal nerve, which joins 

 the sympathetic filaments in the chest and passes to the heart. In man the 

 depressor nerve is not isolated, but its fibres are contained in the sheath of 

 the pneumogastric. This nerve has a reflex action, as was shown by the ex- 

 periments of Cyon, its Faradization reducing the arterial pressure by one- 

 third or one-half. This action is known to be reflex, for when the nerve is 

 divided, stimulation of the central end affects the arterial pressure, while no 

 such result follows stimulation of the peripheral extremity ; and the effect is 

 manifested when the pneumogastrics have been divided and no direct ner- 

 vous influence is exerted over the heart. It is thought that the reduction in 

 the arterial pressure following stimulation of the so-called depressor nerves 

 is due mainly to the action of the splanchnic nerves, by which the abdominal 

 vessels become largely dilated. If the abdomen be opened and one or more 

 of the splanchnic nerves be divided, the arterial pressure is immediately 

 diminished, and the pressure is restored if the divided ends of the nerves 

 be stimulated. If, after division of the splanchnic nerves and the conse- 

 quent diminution of the arterial pressure, the depressor nerves be stimulated, 

 the pressure still undergoes some additional diminution, but this is much less 



