ANATOMY OF A 17.8 MM. HUMAN EMBRYO 63 



the pulmonary arch (ductus arteriosus). Winding around this 

 arch ventro-dorsally, it extends cephalad. In the neck it has 

 relations similar to those of the corresponding nerve on the right, 

 with the exception that it passes medial to the parathyreoid IV 

 instead of dorsal. 



It has been suggested by Froriep ('85) that the recurrent nerves 

 may well be considered as trunks formed by the fusion of bran- 

 chial nerves for clefts which fail to develop. This interpretation, 

 again advanced by Lewis in 1906, gains support from the relation 

 of the posttrematic rami IV to the recurrent nerves in this em- 

 bryo, especially on the right side where the two nerves a^e brought 

 close together, the caudal aortic arches having atrophied. 



One of the inferior cardiac branches of the right vagus is seen 

 in plate 3. It (N.ci.) arises from the dorso-medial wall of the 

 vagus caudad of the place of origin of the recurrent nerve. It 

 extends caudad in the angle between the oesophagus and the 

 trachea, being dorsal to the vagus and medial to the internal jugu- 

 lar vein. It eventually becomes in part incorporated in the deep 

 cardiac plexus, a part rejoining the vagus. Slightly caudad of 

 the above nerve another inferior cardiac ramus from the vagus 

 extends medially to the anlage of the deep cardiac plexus. It 

 is hidden in the reconstruction by the overlying vagus. An in- 

 ferior cardiac nerve was found on the left side corresponding to 

 the right nerve shown in plate 3, but it has been omitted in the 

 reconstructions. It arises from the left vagus just after it has 

 given off its recurrent ramus, hence at a considerably more 

 caudal level than the right inferior cardiac. It extends caudad 

 in the angle between the oesophagus and the trachea to the deep 

 cardiac plexus. 



N. accessorius. The accessory nerve {N.acc, plates 2 and 3) 

 is form.ed by a series of small rootlets which emerge from the 

 lateral surface of the spinal cord and myelencephalon. The 

 most caudally placed root of the series issues at the level of, and 

 in close proximity to, the dorsal root of the second cervical nerve. 

 In this embryo the place of emergence of the most caudally 

 placed root is similar on the two sides, and has been noted in 



