58 A Manual of Veterinary Physiology. 



injured without provoking the least sign of pain on the part 

 of the animal. Colin's experiments in this direction on 

 horses are most conclusive. Not only is the external 

 surface insensible to pain, but the internal surface also. 

 The experimental introduction of foreign bodies into the 

 cavities of the heart produces no pain. This fact has 

 been previously mentioned when speaking of the cardiac 

 sound. 



These results are confirmed by what we know of patho- 

 logical processes ; those foreign bodies found so commonly 

 in the heart of the cow are remarkable for the fact that they 

 produce no suffering, though great disturbance of the circu- 

 lation occurs. 



The nerves supplying the heart are the pneumogastric 

 and sympathetic. The latter has very extensive connections 

 with various ganglia before it reaches the heart. The 

 function of these two nerves is diametrically opposite. 

 Whilst one, the pneumogastric, has a controlling, or, as it is 

 termed, an inhibitory effect over the movement of the 

 heart, the sympathetic has an accelerating or augment- 

 ing effect. Histologically the two nerves differ greatly in 

 structure, the pneumogastric being a medullated, whilst the 

 sympathetic is a non-medullated nerve. 



If the vagus nerve in the neck be gently excited, the 

 heart's beats are reduced in force and freq a< ncy ; if strongly 

 stimulated, the heart stops in diastole. If the nerve be 

 divided the heart beats more rapidly, for now the in- 

 hibitory power over the sympathetic is lost, and the latter 

 has it all its own way. When the cut end of the pneumo- 

 gastric is stimulated we restore to an extent the inhibitory 

 power, and the heart's beats become fewer and more feeble. 

 If the spinal cord and both sympathetics be divided, the 

 inhibitory power over the heart produced by the vagus is 

 intensified, owing to the loss of its antagonistic nerve, and 

 the result is that even feeble stimulation of its fibres arrests 

 the action of the heart. 



The sympathetic is the augmenting or accelerating nerve 

 of the heart; and as such is the antagonist of the vagus. 



