286 ESSENTIALS OF PHYSIOLOGY. 



which increase the sensitiveness of the centre to carbonic acid (p. 275) ; 

 and (2) by the chemical stimulus of the additional carbonic acid pro- 

 duced in the muscles. In addition, the respiratory centre is stimulated 

 by lactic acid, the amount of which in the blood increases during 

 muscular exercise, if the latter is at all severe. During rest, the blood 

 in man contains only minute traces of lactic acid, and this is not in- 

 creased by moderate exercise, such as walking, though running for a 

 few minutes raises the amount of lactic acid in the blood, and lactic 

 acid may be detected in the urine. Notwithstanding the great increase 

 in the supply of oxygen to the actively contracting skeletal muscles, the 

 latter are unable to oxidise all the lactic acid which is formed in them in 

 these circumstances. The excess of lactic acid passes into the circula- 

 tion, and not only stimulates the respiratory centre, but also (p. 258) 

 renders the blood more readily dissociable and enables the muscles to 

 obtain oxygen more easily. 



The changes in the circulation during exercise are acceleration of 

 the heart and a rise in the mean arterial pressure, associated with 

 dilatation of the blood-vessels to the muscles. The acceleration of the 

 heart is due primarily to diminution in the tone of the vagus centre 

 and, therefore, of the restraining influence which it normally exerts 

 on the heart. It is seen at the very beginning of exercise, being 

 brought about in all probability by impulses passing from the cerebral 

 cortex to the vagus centre, and is particularly well marked in animals, 

 such as the dog, which are accustomed to take severe exercise. 

 Subsidiary factors which contribute later to the acceleration are (1) 

 an increase in the tone of the accelerator centre, and (2) the setting 

 free of adrenalin into the circulation. At the same time, the increased 

 respiratory movements and the muscular movements increase the 

 amount of blood reaching the heart, and its output becomes larger. 



The rise in the arterial blood pressure is due partly to the increased 

 output of the heart, and partly to constriction of the splanchnic 

 vessels ; and, since the blood-vessels to 'the muscles are dilated, the 

 flow of blood is largely diverted from the abdominal organs to the 

 skeletal muscles. 



Owing to these factors, the velocity of the blood flow through 

 the lungs, and indeed through the whole body, becomes greater ; and 

 within a given time a much larger quantity of oxygen can be taken 

 up by the blood as it passes through the lungs, and can be transported 

 to the tissues, than during rest. 



During exercise a large amount of heat is evolved in the muscles, 

 and the temperature of the body may rise to 100 or 101 F. ; this rise 

 in temperature tends further to accelerate the heart, and is one of 



