518 



VERTEBRATE LIFE AND ORGANIZATION 



f 



First s-waJlo-wing 



Chews CTxd. 





S<z-cond swallo-winO 

 Esop"ha.^uS 

 Reticulum 



Diaphragra 



To intestine 



Figure 26.3. Course of food through the "stomach" of a cow. Only the abomasum 

 represents the true stomach. The other chambers are derived from the esophagus. 



218. The Pharynx and Esophagus 



Part of the pharynx ot man lies above the soft palate (Fig. 26.2) 

 and receives the internal nares and the openings ol the pan- ot Eu- 

 stachian tubes from the middle ear cavities. Another part lies beneath 

 the soft palate and is in communication with the mouth cavity. The 

 rest of the pharynx lies posterior to these parts and leads to the esoph- 

 agus and larynx. Passage of the food into the pharynx initiates a series 

 of reflexes: The muscular soft palate rises to prevent food from entering 

 the nasal cavities, breathing momentarily stops, the larynx is elevated and 

 the epiglottis swings over the glottis to prevent food from entering the 

 larynx, the tongue ^^revents food from returning to the mouth, and mus- 

 cular contractions of the pharynx move the bolus into the esophagus. 



The pharynx of tetrapods is a rather short region in which the 

 food and air passages cross, but in fishes it is a more extensive area 

 associated with the gill slits. Gill pouches are present in the embryos of 

 mammals, and some of them give rise to glandular structures such as 

 the thymus and parathyroids, but only the first two remain as rudiments 

 in adults. The middle ear cavity and the Eustachian tube develop from 

 the first pouch (the spiracle of fishes), and part of the second forms the 

 fossa in which the palatine tonsil lies. The thyroid gland and the lungs 

 are outgrowths from the floor of the pharynx. Glands derived from the 

 pharynx are endocrine in nature and will be considered in Chapter 30. 



Successive waves of contraction and relaxation of the muscles, known 

 as peristalsis, propel the bolus down the esophagus to the stomach. The 

 muscles relax in front of the food and contract behind it. When the 

 food reaches the end of the esophagus the cardiac sphincter, which closes 

 off the entrance to the stomach, relaxes and allows it to enter. 



The esophagus is generally a simple conducting tube but in some 

 animals its structure has been modified for storage. The crop of the 

 pigeon and the three anterior chambers of the cow's "stomach," for 

 example, are modified parts of the esophagus. They are lined with the 

 stratified squamous epithelium characteristic of the anterior parts of 

 the digestive tract, whereas a true stomach and the intestine are lined 

 with a simple columnar epithelium. The "stomach" of the cow and other 

 ruminants consists of a series of four chambers (Fig. 26.3). Food passes 



