Ii 4 A MANUAL OF VETERINARY PHYSIOLOGY 



explanation given of the respiratory rise and fall of blood- 

 pressure. 



If the curves be closely examined, it will be found that during 

 inspiration the pulse frequency is increased, while during ex- 

 piration it is decreased. This increase appears to hold good for 

 all animals, and is considered to be due to a diminished activity 

 of the cardio-inhibitory mechanism, while the reduction in 

 pulse-rate is believed to be caused by a stimulation of the same 

 centre. Any increase in the action of the heart results in more 

 blood being thrown into the aorta, and so raises the blood-pressure, 

 and this increase is supplemented by the aspiratory action of 

 the right auricle in sucking in blood from the veins. So far, 

 then, the general effect of a rise in blood-pressure during inspira- 

 tion can be explained. The next thing to account for is the 

 delay in the rise at the beginning of inspiration, and this is con- 

 sidered to be due to the dilatation of the lungs causing an increase 

 in the capacity of the pulmonary vessels. The result of the 

 increase is that they have to be filled, and the delay thus caused 

 explains the delay in the rise of blood-pressure. In expiration 

 the pulmonary vessels become smaller, and the delayed fall in 

 pressure is attributable to these vessels having not yet returned 

 to their expiratory capacity. 



Such, then, is the explanation accepted by most physiologists, 

 but in recent years the investigation of the phenomena by 

 T. Lewis has resulted in considerable light being thrown on the 

 matter. This observer shows that the respiratory curves of 

 blood-pressure are of very complex origin, and unless the nature 

 of the respiratory act be known, it is not possible to say what 

 the effect on the blood-pressure will be. In a man trained to 

 breathe with his ribs, the blood-pressure falls during inspiration 

 and rises during expiration ; when abdominal breathing is 

 practised, there is a rise during inspiration and a fall during 

 expiration ; and with costal and abdominal respiration combined, 

 intermediate results are obtained. 



The secondary inspiratory rise of blood-pressure which follows 

 the fall, where respiration is slow and the costal type of 

 breathing is present, is, according to Lewis, due to lessened 

 intrapleural pressure, therefore lessened pressure on the heart, 

 and in consequence an increased filling of the organ. He was 

 able to show, by introducing or withdrawing air from the peri- 

 cardial cavity, how sensitive the heart is to an increase or reduc- 

 tion of intrapericardial, and therefore of intrapleuraTpressure. 



The Nostrils.— Before the air reaches the lungs if is warmed 

 by passing through the nasal cavities, so that -it" eaters the 

 trachea at nearly tKe body temperature. The incoming air also 

 becomes saturated with watery vapour ; this saturation likewise 



