666 SPECIAL PHYSIOLOGY. 



tion, and continues to increase during expiration. Moreover, by these 

 researches, in which a very large haemadynamometer and kymogra- 

 phion were used, the respiratory act is shown to consist of a period of 

 action occupying two-fifths, and of a period of repose occupying three- 

 fifths of the entire act ; of the former period, two-thirds are taken up 

 by inspiration, and one-third by expiration. The arterial pressure 

 begins to increase at the commencement of inspiration, and continues 

 to rise during expiration, at the end of which, and during the pause, it 

 gradually subsides. In violent expiration, the vascular tension is 

 increased ; but also in the prolonged inspiratory efforts of dyspnoea. 



The increased pressure from expiration is illustrated in the tension, 

 and occasional rupture, of bloodvessels in the act of coughing. It has 

 already been mentioned that the velocity of the blood in the arteries 

 is slightly increased during expiration, and diminished during inspira- 

 tion, contrary to what happens in regard to the blood pressure. In- 

 deed all the conditions connected with increased resistance by friction, 

 which diminish the velocity, increase the blood pressure ; whilst those 

 which lessen the friction and resistance, diminish the pressure and in- 

 crease the velocity. All variations in the arterial blood pressure, are 

 less marked, when the pulse is more frequent, and also as the arteries 

 become smaller. 



A double haemadynamometer, or differential manometer, has been 

 devised by Bernard, by means of which the different degrees of pres- 

 sure in different arteries, or in the same artery on the two sides of the 

 body, under different conditions, or the different pressure in the arte- 

 ries and veins, can be very conveniently determined. It consists of a 

 U-shaped tube, the bend of which is occupied by mercury, with a solu- 

 tion of carbonate of soda, above it, in each leg : to the two extremi- 

 ties, the bloodvessels to be experimented on are attached by suitable 

 pipes provided with stopcocks. When these are opened, if the pres- 

 sure in the two attached bloodvessels is equal, the level of the mercury 

 in each side of the bend remains unaltered ; and when it is unequal, 

 the mercury falls in the leg connected with that vessel, which has the 

 greatest pressure on its contents. For example, in the two carotids, 

 or two facial arteries of a horse, the pressure is equal ; but if the in- 

 strument be connected with one artery near the heart, and with another 

 more remote, it is unequal. Moreover, when this instrument is con- 

 nected with the same artery on the two sides of the body, division of 

 the sympathetic nerves on the one side, is followed by an elevation of 

 the mercury on that side, indicating a loss of tension in the coats of 

 the corresponding vessel. 



The Pulse. 



The pulse is the well-known beat of an artery, sometimes visible to 

 the eye, if the artery be superficial, but more commonly felt by the 

 finger placed upon the beating vessel. Sometimes the pulse is per- 

 ceptible to the individual himself, being either felt as a throbbing sen- 

 Cation, or heard as a noise, when near the ear. The remote cause of 

 the pulse, is the force of the heart, for its beats correspond in number 

 with the contractions of the left ventricle. Its immediate cause, how- 



