THE RABBIT. 109 



does to the heart and stomach, that it is called the vagus 

 (wanderer), or pneumogastric. It leaves the skull along 

 with ix., and runs down the neck and along the oesophagus. 

 Its principal branches and their distribution are as follows : 



(1) Superior laryngeal, given off opposite the larynx and 

 running straight to its distribution in that organ. 



(2) Cardiac depressor, given off at the same point as the 

 above, running down the neck parallel to the main trunk 

 of the vagus, but passing on the dorsal side of the great 

 arteries while the main trunk passes ventrally. Distributed 

 to the heart. 



(3) Recurrent laryngeal. This branch is doubly interest- 

 ing as a very pretty case of the determination of a nerve's 

 course by other considerations than the needs of the animal, 

 and as the only important instance of asymmetry in the 

 rabbit's whole nervous system. It is therefore worth 

 dwelling on at greater length than the other branches. The 

 distribution of this branch is to the larynx, but instead of 

 going direct to its destination, the fibres which compose it 

 go by a most roundabout way. They run in the main 

 trunk as far as the level of the subclavian artery on the 

 right side, and still farther on the left side, namely to the 

 level of the ductus arteriosus. At these points they separate 

 into distinct branch-nerves and, looping under the great 

 arteries, retrace their course along the neck to the larynx. 



The explanation both of the curious course and of the 

 asymmetry is to be found in the vascular system. When 

 the heart is first developed in the embryo rabbit, it lies just 

 beneath the throat, as it does permanently in all fishes. 

 The two nerves to the larynx thus happen to run, one in 

 front of, the other behind the great arterial arches. As 

 the heart shifts its position into the thorax, and the neck 

 lengthens, the first nerve (superior laryngeal) is unaffected ; 

 but the second becomes drawn out into a long loop, which 

 is longer on the left side than on the right because the 

 arches persist on the former but not on the latter side 

 (% 59). 



(4) A direct branch to the heart, given off as the main 

 trunk passes behind the precaval vein. 



(5) A direct branch to the lung. 



