336 ANIMAL HEAT. 



on the heart is just the same as when the thorax is closed. 

 These results are sufficient to show the pulmonary distension 

 and acceleration of the contraction of the heart, stand in the 

 relation to each other of cause to effect. That the influence of 

 the former on the latter is exercised through the nervous sys- 

 tem, and consequently through the vagi (these being the only 

 known channel by which the lungs are in communication with 

 the nervous centres) is sufficiently obvious. Accordingly, we 

 should expect that if this channel were interrupted the effect 

 would be annulled, and experiment proves that it is so. The 

 demonstration is, however, very difficult, for in the dog the 

 pulsations of the heart are already so rapid after section of the 

 vagi that no further acceleration is possible ; a negative result, 

 therefore, would mean nothing. Hering has met this difficulty 

 by carefully exciting the peripheral end of one of the divided 

 nerves after section of both (using Helmholtz's modification), 

 so as to reduce the frequency of the heart's action, and repeat- 

 ing the pulmonary distension under these altered conditions ; 

 the result was still negative. These experiments teach us two 

 important facts relating to the innervation of the lungs, viz., 

 that the pulmonary branches of the vagus contain afferent 

 fibres, the excitation of which by pulmonary distension tends 

 to weaken or paralyze both theinspiratory and cardiac centres 

 in the medulla oblongata ; the one action showing itself in the 

 complete cessation of the rhythmical efforts of the inspiratory 

 muscles, the other in the shortening of the diastolic intervals 

 of the heart. The subject requires much fuller investigation 

 than it has yet received. 



CHAPTER XVIII. 

 ANIMAL HEAT. 



THE temperature of the body is dependent on the relative 

 activity of two sets of processes, viz. : those by which heat is 

 produced or generated, and those by which it is destroyed or 

 lost. The subject admits of being correspondingly divided 

 into two parts the study of the processes, and the study of 

 the resulting state. The former is based on the measurement 

 of the quantity of heat set free at the surface during a given 

 period (Calorimetry) ; the second on the measurement of the 

 temperature existing in the circulating blood and the tissues 

 at the moment of observation (Thermometry). 



