3io VETERINARY PHYSIOLOGY 



act of swallowing, causes an instant arrest of the respiratory 

 movements either in inspiration or expiration. The advantage 

 of this in preventing the food as it is swallowed from passing 

 into the trachea is obvious (fig. JL41, d and d'). 



Stimulation of the Cutaneous Nerves stimulates the in- 

 spiratory centre and causes a deep inspiration. This is seen 

 when cold water is dashed upon the skin, and is more clearly 

 demonstrated in animals with the vagi and upper nerve 

 tracts cut across. In such animals if the skin be touched 

 an inspiratory movement is made. 



The temperature of an animal also acts on the respiratory 

 centre. Increase in temperature accelerates the rate of the 

 heart so, too, it accelerates the rate of the respirations, and 

 in about the same proportion, as is seen in feverish attacks, 

 where pulse and respiration are proportionately quickened so 

 that their ratio remains unaltered. When the respiratory 

 rate rises out of proportion to the rate of the pulse, it is 

 usually an indication that some pulmonary irritation is present. 



VII. Interaction of Circulation and Respiration. The lungs 

 and heart being packed tightly together in the air-tight thorax, 

 and both undergoing periodic changes, necessarily influence 

 one another. At the same time, the close proximity of the 

 respiratory and cardio-motor centres in the medulla seems 1 

 to lead to the activity of one influencing the other. 



Influence of Respiration on Circulation. The circulation 

 is modified in two ways by respiration. First, the rate of 

 the heart, and second, the arterial blood pressure undergo 

 alterations. 



\st. Rate of Heart. If a sphygmographic trace giving the 

 pulse waves during the course of two or three respirations be 

 examined, it will be found that during inspiration the heart is 

 acting more rapidly, while during expiration its action is slower. 



If the vagus be cut these changes are not seen, showing 

 that the inspiratory acceleration is not the result simply of 

 the larger amount of blood which enters the heart during 

 inspiration-, but is really due to changes in the cardio-motor 

 centre the accelerating part of which has its activity in- 

 creased during inspiration, while the inhibitory part is more 



