222 HISTOLOGY 



but many others are fused in indefinite masses. The lymphoid tissue 

 constitutes the bulk of the tonsil. 



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

 sends trabecular prolongations. It contains many blood and lymphatic 

 vessels, together with branches of the glossopharyngeal nerve and 

 spheno-palatine ganglion which supply the tonsil. It contains also the 

 secreting portions of small mucous glands, some of which empty into the 

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

 rounding the tonsil. They resemble other mucous glands of the mouth 

 which will be described presently. Beyond the submucosa is striated 

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

 of the pharynx; striated muscle fibers are therefore readily included in 

 sections of the tonsil. 



The pharyngeal tonsil is an accumulation of lymphoid tissue on the 

 median dorsal wall of the pharynx, between the openings of the auditory 

 tubes (Fig. 207). In childhood it is liable to become irregularly enlarged 

 so as to obstruct the inner nasal openings, thus forming the "adenoids" 

 of clinicians. It is covered with stratified epithelium, which is ciliated 

 in embryonic life; and in the adult, cilia may be found upon the epithelium 

 within the pits. The pits and lymphoid tissue are quite like those of 

 the palatine tonsils. 



The lingual tonsil is an aggregation of pits surrounded by lymphoid 

 tissue (Fig. 210). It is found in the back part of the tongue (Figs. 207 and 

 220), the surface of which is very different in texture from the front part, 

 presenting low mounds with central depressions. Each depression is the 

 outlet of a pit. Lymphocytes pass through the epithelium (Fig. 211) 

 and become salivary corpuscles, which are said to produce substances 

 protecting the tissue from bacterial invasion. 



THYMUS. 



The thymus (Gr. 6vtw>, thymus) arises from the two tubular prolonga- 

 tions of the third pharyngeal pouches, which meet in the median line as 

 shown in Fig. 208, 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. Haller (1761) 

 described it in older embryos as "a huge gland, scarcely smaller than the 

 kidney; but in the adult it is diminished, and having become constricted, 

 dried up and much harder, it is almost buried in the surrounding fat." 

 Meckel found ordinarily no trace of it at twelve years, and according to 



