ANATOMY OF A 17.8 MM. HUMAN EMBRYO 61 



Froriep ('85) has shown conclusively that the superior laryn- 

 geal nerve is the posttrematic ramus of the third branchial 

 cleft. 



Another branch {R.p-tr.) of the left vagus arises from the 

 ganglion nodosum directly caudad of the place of origin of the 

 superior laryngeal. It is a small strand which extends caudally, 

 at first between the vagus and the lateral lobe of the thyreoid 

 gland, and dorsal to the thymus and left common carotid artery. 

 Having passed the caudal extremity of the thymus gland it is in 

 relation dorsally with the vagus, medially with the aortic arch, 

 and laterally with the internal jugular vein. The nerve finally 

 winds around the fourth aortic arch, and extends cephalad lateral 

 to the recurrent of that side. It passes dorsal to the parathy- 

 reoid of the fourth pharyrgeal pouch and turning cephalad, 

 appears at first dorsal and then dorso-medial to the lateral lobe 

 of the thyreoid gland. It ultimately becomes exhausted by giv- 

 ing off fibers which extend ventrally in the region of the termina- 

 tion of the external ramus of the superior laryngeal nerve. 



The corresponding nerve on the right takes origin from the 

 dorso-medial side of the right ganglion nodosum, where the hypo- 

 glossal nerve crosses the vagus laterally. As it extends caudad 

 it lies close to the medial side of the vagus, and it is conceivable 

 that in later development the two nerves might be inclosed in a 

 common connective tissue sheath. It finally winds around the 

 right fourth aortic arch (A. subclavia) in the concavity of the 

 larger recurrent nerve, and returns cephalad, passing lateral to 

 the parathyreoid of the fourth pharyngeal pouch, but having the 

 same relation to the lateral lobe of the thyreoid as the corre- 

 sponding nerve on the right. It has a shorter course in the neck 

 than that of the right side, the right fourth aortic being situated 

 considerably more cephalad than the left. 



From their site of origin, and from their relation to the fourth 

 aortic arches and to the parathyreoids of the fourth pouches, it 

 can scarcely be doubted that these nerves represent the post- 

 trematic rami of the fourth pharyngeal pouches. 



These nerves are obviously comparable to the branches of the 

 vagus identified by Froriep ('85) in cow embryos (8.7 to 8.8 mm. 



