BY DR. BURDON-SANDERSON. 279 



paper, soaked with water. The " moist chamber" so prepared 

 is immersed vertically in a test tube filled with cold water, 

 which also contains a thermometer. The water in the beaker 

 is then very gradually warmed, while its temperature and the 

 frequency of the contractions of the heart are noted from time 

 to time. It is then seen that the frequency gradually increases 

 up to about 34 C., above which the contractions become ir- 

 regular, and are difficult to count with exactitude, until at last 

 the condition known as " heat rigor" (with reference to which 

 see Chapter XX.) supervenes. Similar observations may be 

 made with respect to portions of the heart, as, e. g., the base 

 of the ventricle or the sinus venosus. For this purpose it is 

 convenient to place the fragment on a cover glass in a drop 

 of serum, and invert it over the chamber of Strieker's warm 

 stage. 



72. (b) In Mammalia. From the observation of the very 

 remarkable effects which diminution and increase of the in- 

 ternal temperature of the bod\ T respectively produce, the one * 

 in diminishing, the other in increasing, the frequency of the 

 pulse in rabbits and dogs, it seems probable that the mammalian 

 heart is more sensitive to temperature changes than that of the 

 amphibia. As, however, it is not possible to eliminate the in- 

 fluence of the central nervous system, this cannot be proved 

 experimentally. 



SECTION VIII. THE INHIBITORY NERVES OP THE HEART. 



73. 1. Demonstration of the Influence of the Vagus 

 Nerve on the Heart in the Frog. Description of the 

 Vagus Nerve. The vagus nerve originates in the frog from 

 the posterior aspect of the medulla oblongata by three or four 

 roots, the lowest (analogous to the spinal accessory) being 

 more to the front than the rest. The nerve passes out of the 

 cranial cavity through the condyloid foramen of the occipital 

 bone, outside of which it forms a ganglion, and is in close 

 relation with the sy in pathetic trunk. After leaving the sym- 

 pathetic (see fig. 237). it divides into two branches, of which 

 the anterior contains the glossopharyngeal, the posterior the 

 nerves which are distributed to the heart, lungs, and other 

 viscera. The vagus itself and its cardiac branch run along- 

 side of and in the same direction with the lower of the three 

 petrohyoid muscles, as far as the extremity of the posterior 

 horn of the hyoid bone, into which the muscle is inserted. 

 During this part of its course it is accompanied by the laryn- 

 goal nerve, which leaves it just before it reaches the insertion 

 of the muscle. At about the same point it crosses the apex of 

 the lung, passing behind the pulmonary artery, and gives off 

 pulmonary branches which accompany that vessel. Having 



