1138 DIGESTIVE SYSTEM 



Vessels and nerves. — The vessels of the tonsil and the motor nerves of the various muscles 

 have already been mentioned. In general, the arteries to the pharynx are derived chiefly from 

 the ascending pharyngeal, the ascending palatine branch of the external maxillary, and the 

 descending palatine and pterygo-palatine branches of the internal maxillary. The veins 

 form a venous plexus between the pharyngeal constrictors and the pharyngeal aponeurosis, 

 and also an external plexus, communicating with the pterygoid plexus above and with the 

 posterior facial or internal jugular vein below. The lymphatic vessels pass chiefly to the 

 deep cervical nodes, those from the upper portion (including the pharyngeal tonsil) ending 

 partly in the retro-pharyngeal glands. The nerves of the pharynx, both motor and sensory, are 

 derived chiefly from the glosso-pharyngeal and vagus, by way of the pharyngeal plexus. 



The development of the pharynx. — The pharynx is developed chiefly (if not entirely) from 

 the anterior end of the archenteron. In this portion of the archenteron, with the develop- 

 ment of the branchial arches, there are formed on each side four entodermal pouches or grooves 

 (with a rudimentary fifth), the branchial clefts (see p. 17). With further development the first 

 pair of branchial clefts form the tympanic cavities and the auditory or Eustachian tubes; the 

 lower portion of each second branchial cleft persists as a fossa in which a palatine tonsil is 

 developed; the remains of the third and fourth pairs are found on each side in the vallecula 

 and piriform sinus of the larynx. The origin of the pharyngeal tonsil may be observed in the 

 third month of foetal life in the form of small folds of mucous membrane which, during the 

 sixth month, become infiltrated with diffuse adenoid tissue, lymph-nodules differentiating in this 

 toward the end of foetal life. The pharyngeal bursa, which is not a constant structure (Kil- 

 han), may be observed as a small diverticulum of the pharyngeal wall, closely connected with 

 the anterior extremity of the notochord. The diverticulum develops independently of Rathke's 

 pouch (which gives rise to the anterior portion of the hypophysis), and is also apparently distinct 

 from Seesel's pocket. 



The entire pharynx, like the associated facial region, is relatively small and undeveloped 

 in the fcetus and newborn, but develops rapidly during infancy. The development of the 

 muscles and of the palatine tonsils has already been considered. 



Variations. — Variations in the palatine and pharyngeal tonsils and in the pharyngeal bursa 

 have already been mentioned. Remnants of the visceral clefts may persist as aberrant diver- 

 ticula or as 'branchial fistulse' connected with the pharynx. Many additional muscles have been 

 described, chiefly longitudinal muscles arising from the base of the cranium either by split- 

 ting of those normally present, or as separate slips. A detailed description of these may be 

 found in Poirier-Charpy's work. Abnormally extensive fusion of the posterior arches of the 

 palate with the walls of the pharynx may produce a congenital stenosis of the pharyngeal isthmus. 



Comparative. — The pharynx is not distinctly separated from the mouth cavity in the 

 lower vertebrates. It is the region containing the branchial or visceral clefts and is thus both 

 respiratory and ahmentary in function. The nasal pharynx, including the apertures of the 

 auditory tubes, becomes distinct along with the nasal cavity when the palate is formed (from 

 the reptiles upward). In the air-breathing vertebrates, the laryngeal aperture appears in 

 the ventral wall of the pharynx just anterior to the beginning of the oesophagus. Of the tonsils, 

 the pharyngeal are the most primitive, being present in the roof of the pharynx in amphibia, 

 well-developed in reptiles, birds, and mammals (Killian). The palatine tonsils, on the other 

 hand, are characteristic of mammals, being rarely absent, however (e. g., rat, guinea pig). 

 From the embryological point of view, Hammar has classified the palatine ^tonsils in the 

 various mammals under (1) the primary type (including rabbit, cat, and dog), in which the 

 tonsil is formed from the embryonic tonsillar tubercle (described above under development 

 of tonsil); and (2) the secondary type (including pig, ox, sheep and man), in which the tonsil- 

 lar tubercle disappears and the tonsil is developed from the wall of the surrounding tonsillar 

 sinus. Typical epithelial crypts (highly branched in the ox) are found only in the secondary 

 type. The tonsil may form a single (lymphoid) lobe (cat, pig, rabbit) or may develop typi- 

 cally two lobes (ox, sheep, man), separated by the intratonsillar fold. There are great varia- 

 tions among different species as to relative size, number and character of folds, crypts, 

 etc. The intimate relation of the epithelium with the underlying lymphoid tissue is charac- 

 teristic and constant. 



THE (ESOPHAGUS 



Tlio (esophagus (figs. 895, 896) is that portion of the ahmentary tract which 

 extends between the pharynx and the stomach. It is more constricted than the 

 rest of the canal, being narrowest at its commencement opposite the lower 

 border of the cricoid cartilage. It is again somewhat contracted behind the left 

 bronchus, and at its passage through the diaphragm, which is opposite the tenth 

 or eleventh thoracic vertebra. It has an average lengtii of 25 cm. (varying from 

 20 to 35 cm.). The average distance from the rima oris to the beginning of the 

 oesophagus is about 15 cm. In its course downward the oesophagus follows the 

 curves of the vertebral column until it finally pass(^s forward in front of, and 

 slightly to the left of, the aorta to gain the oeso])hageal opening in the diaphragm. 

 In addition to these curves it ])resents two lateral (un'vatures, one convex toward 

 the left sific! at th(! root of the neck and in the upper part of the thorax, and the 

 other concave toward the left in the lower part of the thorax where it leaves the 

 vertebral column. It lies in the middle line at its commencement (usually 

 opposite the sixth cervical vertebra), and again, at a lower level, opposite the fifth 

 thoracic vertebra. 



