334 AN INTRODUCTION TO ZOOLOGY 



lacerum posterius in company with the tenth and eleventh nerves 

 and the internal jugular vein. It passes downwards between the 

 jugular vein and the internal carotid artery and then turns forward 

 to spread out in the tongue, tonsils and pharyngeal region. 



As we have seen, the vagus nerve leaves the skull through the 

 foramen lacerum posterius, and it swells out shortly after to form a 

 distinct vagus ganglion. It descends the neck external to the 

 common carotid artery into the thoracic cavity, and then through 

 this alongside the oesophagus into the abdomen, where in proximity 

 to the stomach it breaks up. Just beyond the ganglion at the 

 anterior end of the neck it gives off the anterior laryngeal nerve, a 

 small trunk running inwards and forwards dorsal to the carotid 

 artery, to be distributed to the larynx and the crico-thyroid muscle. 

 A second branch, the depressor nerve, arises generally from the 

 preceding, but it may come directly from the main trunk at or near 

 the same point. This passes back along the neck mesial to the 

 main vagus trunk and dorsal to the common carotid artery to go 

 to the heart. It receives its name from the fact that stimu- 

 lating it produces a depression of the beating of the heart. 

 The last noticeable branch from the vagus is the posterior or 

 recurrent laryngeal nerve, which is differently disposed on the two 

 sides. On the right it leaves the vagus just in front of the sub- 

 clavian artery, then loops around this vessel and finally takes up a 

 position at the side of the trachea and passes forward along it to 

 supply most of the laryngeal muscles. On the left side it comes off 

 further down, behind the sub-clavian artery and actually in the 

 thoracic cavity. It loops around the arch of the aorta and so 

 reaches its position by the side of the trachea. This curious looping 

 of the nerves is comprehensible when we consider the development 

 of this region. Originally the heart is situated much more anteriorly, 

 and its carotid and systemic trunks, representing the third and 

 fourth of the afferent branches of a fish, e.g. Scyllium, are morpho- 

 logically in front of the point of origin of the larynx. The nerves 

 then run behind these arteries. In the course of development the 

 heart, and with it the arteries, shifts backwards and consequently 

 the nerve becomes pulled out into a loop. 



While traversing the thorax the main trunk of the vagus gives off 

 branches to the lungs, oesophagus and heart, the last named forming 

 a plexus around the roots of the aorta and pulmonary artery. 



The spinal accessory nerve is a short nerve also leaving the 

 cranium by the foramen lacerum posterius. It passes almost 

 vertically downwards lateral to the vagus, and is distributed to the 

 sterno-mastoid and other adjacent muscles. A short distance 

 along it gives off a ramus internus which joins the vagus. 



