148 STUDENTS HISTOLOGY 



(They resemble the parotid elements, although smaller and less 

 granular. The acini are more tubular than in the parotid gland. 

 Even with the low-power one may distinguish small, rounded areas 

 called the bodies of Langerhans. They are probably groups of 

 immature acini.) 



(<Z) In the smaller ducts of the parotid and submaxillary glands, 

 notice that the lining epithelial cells have vertical striations at the 

 outer border. 



The sublingual gland in man is an almost purely mucous 

 gland. The parotid is purely serous, and has a secretion of that 

 character. It is reckoned as a true salivary gland. The submax- 

 illary, having elements of both kinds, is called mixed. In the 

 serous glands, when the cells are filled with secretion, they appeal- 

 large, with fine granules. After discharge of the secretion they are 

 smaller, dark, and granular. The cells of the mucous glands may 

 be found large and clear, or after discharge of the mucus, also 

 smaller, dark, and granular. At the borders of the acini of the 

 mucous glands there may be seen crescent -shaped groups of granu- 

 lar cells, the demilunes .of Heidenhain. 



TEE THYROID GLAND 



The thyroid gland is a compound tubular gland. The tubular 

 acini are 40 to 120 p- in diameter. Loose connective tissue unites 

 them into lobules and lobes, and forms a covering for the whole. 

 The acini are closed cavities. They are lined by low columnar or 

 cubical epithelial cells. The cells are limited by a basement mem- 

 brane. The acini are usually filled with homogenous, yellow, 

 translucent material, called the colloid substance, which is formed 

 by the epithelial cells. The blood supply is abundant, and a rich 

 capillary network surrounds the acini. 



The lymphatic network is also profuse, and the characteristic 

 colloid substance may be found within the lymphatics. Nerve- 

 fibers are not numerous. They are mostly non-medullated, derived 

 from the sympathetic. Although the thyroid gland of the adult 

 has no duct, in the embryo it has one the thy ro- glossal duct, 

 which has an opening corresponding to the foramen caecum, near 

 the -base of the tongue. Later on, this duct becomes obliterated 

 .for the most part. 



