62 • F, W. THYNG 



in length) as the posttrematic rami of the fourth pharyngeal 

 pouches, and figured in Taf. I, figs. I and II, II, 115. Froriep, 

 however, could find this nerve only in the young stages. 



Lewis ('06) found in the 12 mm. pig embryo a small nerve 

 running beside the postbranchial body which he thought might 

 be comparable to the nerve described by Froriep as the post- 

 trematic ramus for the fourth pouch. 



Elze ('07) identified this posttrematic branch of the vagus 

 in a human embryo of about 7 m_m., (p. 427, text figs. 7-8), but 

 could not find the nerve in two older human embryos (II and III), 

 measuring (greatest length) 9.5 and 11 mm. respectively. Hence, 

 like Froriep, he concluded that in man the existence of this post- 

 trematic ramus of the vagus is transitory. 



In this embryo it is seen that these posttrematic rami become 

 closely associated with the recurrent, especially on the right 

 side where the pulmonary aortic arch atrophies, and the author 

 believes that they occasionally, at least, persist in the adult. 

 The support for this conclusion is based not only upon their 

 presence in an embryo of this stage of development (17.8 mm.), 

 but upon the observations of Wrisberg. Wrisberg (Henle's 

 Anatomie des Menschen, Bd. 3, p. 441, 1868) observed in three 

 cases a reduplication of the right recurrent nerve. The extra 

 branch was much smaller than the normal, and it accompanied 

 the latter upwards between oesophagus and trachea, in much the 

 same way as occurs in this embryo. 



The recurrent nerves are displayed in plate 6. The right recur- 

 rent nerve (A^rec), arises from the vagus at the caudal border 

 of the right fourth aortic arch (A. subclavia), and passes ceph- 

 alad in the neck, dorsal to the parathyreoid of the fourth pharyn- 

 geal pouch (P.-thyr.IV). Here it gives off some oesophageal 

 branches, and then continues to the medial side of the right 

 lamina of the thyreoid cartilage where it becomes exhausted by 

 giving off branches which turn ventrally to the anlagen of the 

 laryngeal muscles. 



The left recurrent nerve (N.rec.s.) arises from the vagus at 

 a more caudal level than the right, viz., at the caudal border of 



