THE BRANCHIAL REGION 



67 



ably arises in the upper part of this arch. The cartilage of the third arch gives origin 

 to the greater cornu of the hyoid bone. The ventral ends of the second and third 

 arches unite with those of the opposite side, and form a transverse band, from 

 which the body of the hyoid bone and the posterior part of the tongue are devel- 

 oped. The ventral portions of the cartilages of the fourth and fifth arches unite 

 to form the thyroid cartilage; from the cartilages of the sixth arch the cricoid 

 and arytenoid cartilages and the cartilages of the trachea are developed. The 

 mandibular and hyoid arches grow more rapidly than those behind them, with 

 the result that the latter become, to a certain extent, telescoped within the 

 former, and a deep depression, the sinus cervicalis, is formed on either side of 

 the neck. This sinus is bounded in front by the hyoid arch, and behind by the 

 thoracic wall ; it is ultimately obliterated by the fusion of its walls. 



From the first branchial groove the concha auriculae and external acoustic 

 meatus are developed, while around the groove there appear, on the mandibular 

 and hyoid arches, a number of swellings from which the auricula or pinna is formed. 

 The first pharyngeal pouch is prolonged dorsally to form the auditory tube and the 

 tympanic cavity; the closing membrane between the mandibular and hyoid arches 



Membranous capsule over cerebral hemisphere 



Fronto-nasal process 



Stomodeum 



Lateral naml process 



Eye 



Globular process 

 Maxillary process 



Fio. 44. Under surface of the head of a human embryo about twenty-nine days old. (After His.) 



is invaded by mesoderm, and forms the tympanic membrane. No traces of the 

 second, third, and fourth branchial grooves persist. The inner part of the second 

 pharyngeal pouch is named the sinus tonsillaris; in it the tonsil is developed, above 

 w r hich a trace of the' sinus persists as the supratonsillar fossa. The fossa of Rosen- 

 muller or lateral recess of the pharynx is by some regarded as a persistent part of 

 the second pharyngeal pouch, but it is probably developed as a secondary forma- 

 tion. From the third pharyngeal pouch the thymus arises as an entodermal diver- 

 ticulum on either side, and from the fourth pouches small diverticula project and 

 become incorporated with the thymus, but in man these diverticula probably 

 never form true thymus tissue. The parathyroids also arise as diverticula from 

 the third and fourth pouches. From the fifth pouches the ultimobranchial bodies 

 originate and are enveloped by the lateral prolongations of the median (hyroid 

 rudiment; they do not, however, form true thyroid tissue, nor are any traces 

 of them found in the human adult. 



The Nose and Face. During the third week two areas of thickened ectoderm, the 

 olfactory areas, appear immediately under the fore-brain in the anterior w r all of the 

 stomodeum, one on either side of a region termed the fronto-nasal process (Fig. 44). 

 By the upgrowth of the surrounding parts these areas are converted into pits, 



