THE SALIVARY GLANDS 139 



alveoli ; in this it is flattened (d'). The columnar epithelium of the 

 ducts is peculiar, in that the cells show a distinction into two unequal 

 zones, an outer, larger, striated zone, and an inner, smaller, granular 

 one (fig. 168, d). 



The cells of the alveoli differ according to the substance they 

 secrete. In alveoli which secrete mucus, such as all the alveoli of the 

 dog's submaxillary, and some of the alveoli of the same gland in man 

 (fig. 170), the cells are clear and swollen out with mucigen, which is 

 discharged into the ducts when the gland is stimulated to activity. But 

 in each alveolus there are some smaller cells which do not contain 

 mucigen, and these generally form crescentic groups which lie next to 

 the basement-membrane (fig. 169, c). These are the so-called crescents 

 Of Gianuzzi ; their constituent cells are also known as marginal cells. 

 In alveoli, on the other hand, which do not secrete mucus, but watery or 

 serous saliva, such as the parotid in all animals, and some of the alveoli 

 of the human submaxillary, the cells are filled with granules when the 

 gland is at rest, although the outer part of each cell may become clear 

 after a long period of secretion (fig. 171). 



The largest ducts have a wall of connective tissue outside the base- 

 ment-membrane, and also a few plain muscular cells. The blood-vessels 

 of the salivary gland form a capillary network around each alveolus. 

 The lymphatics commence in the form of lacunar vessels encircling the 

 alveoli. The nerve-fibres, which are derived both from the cerebro- 

 spinal nerves and from the sympathetic, have not been satisfactorily 

 traced to their termination, but they probably become connected with 

 the alveolar cells. 



THE PHARYNX AND (ESOPHAGUS. 



The pharynx is composed of a fibrous membrane, which is encircled 

 by striated muscles, the constrictors, and lined by mucous membrane. 

 The mucous membrane is lined in the upper part of the pharynx and 

 on the upper surface of the soft palate with ciliated epithelium, which 

 is continuous with that of the nostrils, and through the Eustachian 

 tube with that of the tympanum. Below the level of the soft palate 

 the epithelium is stratified like that of the mouth and gullet, into 

 which it passes. In certain parts the mucous membrane contains a 

 large amount of lymphoid tissue, especially at the back, where it 

 forms a projection which is sometimes termed the pharyngeal tonsil, 

 and there are numerous mucous glands opening on its surface. 



The oesophagus or gullet, which passes from the pharynx to the 

 stomach, consists, like the pharynx, of a fibrous covering, a muscular 

 coat, a lining mucous membrane, and intervening connective tissue 

 (areolar coat) (fig. 172). The muscular coat is much more regularly 

 arranged than that of the pharynx, and is composed of striated muscle 

 in about its upper third only, the rest being of the plain variety. There 



