PALATINE TONSILS. 1 69 



The submucous layer forms a capsule for the organ, into which it 

 sends trabecular prolongations. It contains many blood and lymphatic 

 vessels, together with the secreting portions of the mucous glands, and the 

 branches of the glossopharyngeal nerve and of the spheno-palatine gang- 

 lion which supply the tonsil. Some of the small glands empty into the 

 pits but most of their ducts terminate in the mucous membrane surround- 

 ing the tonsil. They resemble other mucous glands of the mouth which 

 are to be described presently. Beyond the submucosa is striated muscle, 

 belonging to the arches of the palate and to the superior constrictor of the 

 pharynx. 



Except that the palatine tonsils lie in depressions which correspond 

 in position with the second pharyngeal pouches, they afford no evidence of 

 their branchial relations. Only their epithelium is entodermal. The 

 lymphoid tissue is mesenchymal. In these respects the palatine tonsils 

 resemble the median lingual tonsil which forms the posterior part of the 

 tongue (see page 184) and the more diffuse median pharyngeal tonsil on the 

 dorsal wall of the nasopharynx between the openings of the auditory tubes. 

 Irregular enlargements of the latter may obstruct the inner nasal openings, 

 producing the 'adenoids' of clinicians (the adjective adenoid being synony- 

 mous with lymphoid) . The pits of the pharyngeal tonsil are smaller than 

 those of the palatine. 



THYMUS. 



The thymus arises from the two tubular prolongations of the third 

 pharyngeal pouches, which meet in the median line as shown in Fig. 189, 

 and become bound together by their connective tissue coverings. The 

 lumen is lost, and the cells proliferate. They form a broad, flat, bilobed 

 mass with a tapering prolongation up either side of the neck. The bulk 

 of the organ is in the thorax, beneath the upper part of the sternum. At 

 birth it weighs generally between 5 and 15 grams (about half an ounce), 

 and is relatively a large organ. It increases in size and weight for some 

 years after birth, probably until puberty, and then slowly atrophies. At 

 15 years it is said to weigh from 40-50 grams. It is considered an active 

 organ even to the fortieth year, losing its functions with beginning old age 

 (50-60 years). Then it becomes fibrous and fatty. The importance of 

 the thymus has apparently been underestimated. 



The thymus is subdivided by connective tissue layers into lobes from 

 4 to ii mm. in diameter, and these are similarly subdivided into lobules 

 of about one cubic millimeter each. On either side all the lobules are 

 attached to a cord of medullary substance, 1-3 mm. in diameter, as may 



