RESPIRATION BEYOND THE LUNGS 411 



Heart Muscle. The gaseous exchange of the heart has been studied 

 both on isolated heart preparations and by examining the exchange 

 in the lungs of a combined lung and heart preparation. The most 

 important investigations by the first of these methods are those of 

 a known pressure. By altering the initial pressure it was found that the 

 2 used by the heart depends on the product of the pulse frequency and 

 the maximal increase in pressure produced by each cardiac contraction; 



or, in the form of an equation : = a co-nstant quantity ; where Q is 



the oxygen used, T the maximal increase of pressure at each beat, and N 

 the frequency of the pulse. 



It should be pointed out, however, that constancy in the product of 

 the above equation does not hold under abnormal conditions of the heart- 

 beat. For example, when the pressure in the heart is very high, the 

 amount of 2 required begins to go up out of proportion, indicating that 

 the heart is becoming overtaxed that it is losing its efficiency. The 

 same result occurs when the heart is dying, and when depressing drugs 

 are used, such as chloral hydrate, potassium cyanide, veratrine, etc. 

 Some other drugs, however, such as epinephrine, do not cause altera- 

 tion in the ratio, nor does vagus stimulation. Of course when the vagus 

 is stimulated, the 2 consumption in a given period decreases because 

 the heartbeats are slowed ; but the absorption of 2 is not increased rela- 

 tively to the slowing of the heart. 



The oxygen consumption of the heart in a heart-lung preparation (page 

 163) has been investigated particularly by Evans 49 with the object of de- 

 termining the mechanical efficiency of the heart. This involves a com- 

 parison of the actual mechanical work done with the energy expenditure 

 calculated on the basis that 1 c.c. 2 consumed = 2.07 kilo-grammeters 

 of work. It was found that when the pulse-rate is constant and rapid 

 (as it would be in a heart deprived of nerve control), the efficiency be- 

 came greater as the venous inflow was increased. This conforms with the 

 principles laid down elsewhere concerning the so-called law of the heart 

 (page 216). There is every reason for believing that alterations in pulse- 

 rate produced through the nerve control would also influence the effi- 

 ciency, that is, the consumption of 2 is less for a given output of blood 

 per minute when maintained by a slow beat than by a fast one. In the 

 heart-lung preparation alteration in the rate by changing the tempera- 

 ture had this effect. Under the most favorable conditions the mechanical 

 efficiency of the heart was found to be 28 per cent, which is greater than 

 that reached by the body as a whole under the most favorable conditions. 



Glands. Most work has naturally been done on the most accessible 

 gland the submaxillary. By stimulating the secretory nerve of this 



