I270 A MANUAL OF ANATOMY 



part pass to the upper group of deep cervical glands. Some of 

 the lymphatics from the upper part pass to the retropharyngeal 

 gland, two in number, each of Which lies upon the upper part of 

 the corresponding rectus capitis anticus major muscle. 



Development ol the Pharynx, and of the Pharyngeal Pouches or Visceral Clefts 

 and Visceral Arches. 



The pharynx is developed from the anterior or cephalic part of the fore-gut. 



On the ventral wall of this part there are two elevations, the anterior 

 of which is called the tuberculum impar, whilst the posterior is called the 

 furcula. The latter has the form of a horseshoe, the open part of which is 

 placed posteriorly. The furcula is "separated from the tuberculum impar in 

 front, and from the lateral wall of the fore-gut on either side, by a groove 

 called the sinus arouatus. The tuberculum impar gives origin to the anterior 

 two-thirds, or buccal portion, of the tongue ; the anterior part or loop of the 

 furcula gives rise to the epiglottis ; the posterior extremities of the furcula 

 to the arytenoid cartilages ; and the depression within the furcula becomes 

 deepened and developed into the larynx and trachea. Posterior to the 

 furcula the foregut becomes dilated to form the rudimentary stomach, and 

 as this descends the oesophagus is formed. 



Pharyngeal Pouches. 



The pharyngeal pouches (visceral clefts) are situated on each lateral wall 

 of the pharyngeal part of the fore-gut, and are arranged in pairs from the 

 cephalic region caudalwards. Their direction is dorso-ventral, or from 

 behind forwards, and they are almost parallel with each other, there being a 

 sUght convergence ventralwards. The number of pouches on either side is 

 five in the human embryo. The fifth pouch is rudimentary, and is an appen- 

 dage of the fourth, from which it projects in a caudal and ventral direction. 

 The pouches are separated from each other by rounded bars, which represent 

 the visceral arches. In order from the cephalic region caudalwards they 

 are designated as first, second, third, fourth, and fifth pouches. 



Each pouch is developed as an outward protrusion of the entodermic lining 

 of the pharyngeal part of the fore-gut. To meet each entodermic protrusion 

 from the interior an inward extension of the ectoderm takes place from the 

 exterior. In this manner furrows are formed on the exterior of the pharynx. 

 The bottom of each pharyngeal pouch is closed, and is separated from the 

 corresponding external furrow by an epithelial septum, which constitutes 

 the closing membrane. This septum is bilaminar, its interned layer being 

 entodermic, and its external layer ectodermic, both being in intimate contact. 



Metamorphoses of the Pharyngeal Pouches — First Pharyngeal Pouch. — The 

 closing membrane at the bottom of this pouch separates it from the lirst 

 external furrow which corresponds to it. The pouch itself, being pharyngeal, 

 is entodermic, but the external furrow is ectodermic. From this pouch (ento- 

 derm) the tympanitic cavity and the Eustachian tube are developed. The 

 external furrow (ectoderm) becomes the external auditory meatus, and the 

 lips of this furrow become differentiated into the component elements of 

 the pinna. The closing membrane, originally bilaminar, becomes trilaminar, 

 a third layer, consisting of mesoderm, having been developed between the 

 two original entodermic and ectodermic laminae. So constituted, the closing 

 membrane forms the membrana tympanl. 



Second Pharyngeal Pouch. — The remains of this pouch in the adult are — 

 (i) The tonsil; (2) the supratonsillar fossa, a small depression which lie 

 above the tonsil; and (3) the lateral pharyngeal recess or fossa of RosenmulU* 

 behind the cushion of the pharyngeal orifice of the Eustachian tube. 



Third Pharyngeal Pouch. — This pouch disappears, but it furnishes a dive 

 ticulum from which the corresponding lateral lobe of the thymus body 

 developed. 



Fourth Pharyngeal Pouch. — ^This pouch likewise disappears, but, like 

 third, it furnishes a diverticulum from which a portion of the correspondii 



