430 THE RESPIRATION 



formed, with the consequence that the intrathoracic and intraabdominal 

 pressures rise considerably, greatly augmenting the systolic discharge 

 and causing the blood pressure to rise. Because of the obstruction to 

 the bloodflow in the large veins of the abdomen and thorax, however, 

 the later effect of the effort is to diminish the systolic discharge, but the 

 fall in blood pressure which this would be expected to occasion is masked. 

 The pressure remains high because other factors increasing the peripheral 

 resistance come into play. The fall in blood pressure following these acts 

 may be very marked indeed. It may be so marked that fainting occurs 

 because of curtailment of intracranial circulation. Similar mechanical 

 effects are produced in the acts of coughing, sneezing, etc. 



The capacity of the veins varies considerably with the position of the 

 body, and it is in order that we may cause alterations in this capacity 

 and therefore encourage a more rapid bloodflow that we stretch the body 

 after sitting for some time in a cramped position. 



The Nervous Factor. The activity of the vagosympathetic, vasomotor 

 and respiratory centers becomes greatly altered during muscular effort. 

 In the earlier stages the alteration depends on nervous impulses trans- 

 mitted to the centers, but later it also depends on changes in composition 

 and temperature of the blood flowing through them the hormone factor. 

 The stimuli which first act on the centers are derived from the cerebral 

 cortex. They are believed to irradiate on to the medullary centers from 

 the motor pathways along which impulses are passing, on their way down 

 from the cortex to the spinal cord. The most weighty evidence favoring 

 this belief is that increase in the rate of the pulse and respirations may 

 occur at the moment a muscular effort is attempted, before there is any 

 time for hormones to become developed, or for reflexes from the muscles 

 themselves to be set up. Moreover, the degree of alteration of the me- 

 dullary centers is not at first proportional to the actual amount of work 

 done ; if a person expects that much effort will be required to do a piece 

 of work, the pulse and respirations will increase immediately he starts 

 the work, even although this, after all, is trivial. These impulses are, 

 however, incapable of stimulating the respiratory center unless the C0 2 - 

 tension of the blood is normal. After forced breathing, for example, 

 muscular effort does not cause increased breathing. During the progress 

 of the work the cortical influences continue to act on the centers which 

 now are said to be also affected by afferent impulses from the periphery, 

 as well as by hormones. These afferent impulses acting on the pulse rate 

 are supposed by Bainbridge to come from the wall of the ventricle where 

 they are set up by the diastolic tension due to venous inflow. It is 

 difficult to reconcile this view with the slowing of the pulse that occurs 

 in asphyxial and epinephrine hypertension (page 774). 



