STRUCTURE OF SECRETING GLANDS. 



401 



tinctly lobular structure. The lobules are held together by the branches of the 

 duct to which they are appended, and by interlobular connective tissue which also 

 supports the blood-vessels in their ramifications. The larger lobules are made up of 

 smaller ones, these of still smaller, and so on for several successive stages. The 

 smallest lobules consist of a single group of alveoli, collected around a small duct, 

 which issues from the lobule. This issuing duct is often markedly narrower than 

 either the alveoli or the larger duct into which it opens, but since it is lined by 

 somewhat flattened epithelium its lumen is quite as large as that of the alveoli (fig. 

 463, d') ; and a collection of the smallest lobules, united by connective tissue and 

 vessels, forms a lobule of the next size, which, too, has its larger branch of the duct 

 formed by the junction of the ramuli belonging to the ultimate lobules. In this 

 way the whole gland is made up, the number of its lobules and of the branches of 

 its duct depending on its size ; for whilst some glands of this kind, like the parotid, 

 consists of innumerable lobules, connected by a large and many-branched duct, 

 others, such as some of the sebaceous glands of the skin, are formed of but two or 

 three ultimate lobules, or even of a single one. In fact, a small racemose gland 

 resembles a fragment of a larger one. 



The smallest lobules were originally called acini, a term which is now often used 

 to denote the alveoli. 



The alveoli are lined, and sometimes almost filled by the secreting cells, a cavity 

 being left in the centre communicating with the excretory duct (fig. 463). In some 



Fig. 463. SECTION OF A RACEMOSE 



GLAND, SHOWING THE COMMENCEMENT 

 OF A DUCT IN THE ALVEOLI. MAGNI- 

 FIED 425 DIAMETEKS. (E. A. S. ). 



a, one of the alveoli, several of which 

 are in the section shown grouped around 

 the commencement of the duct, d' ; a', an 

 alveolus, not opened by the section ; 

 b, basement-membrane in section ; c, in- 

 terstitial connective tissue of the gland ; 

 d, section of a duct which has passed 

 away from its alveoli, and is now lined 

 with characteristically-striated columnar 

 cells ; s, semilunar group of darkly- 

 stained cells at the periphery of an 

 alveolus. 



glands karyomitotic figures are 

 numerous in the cells, indicating 

 rapid cell-multiplication, whereas 

 in others such indication is lack- 

 ing (Gaule, Bizzozero). In some 



cases, minute canals have been described as leading from the central cavity between 

 the cells, and these may aid in the conveyance of the secretion of the latter into 

 the cavity. Further the flattened cells which compose the basement-membrane 

 may send delicate lamellar processes between the alveolar cells, among which they 

 form a sustentacular network (Boll, Ebiier). 



Many glands, yielding very different secretions, belong to the racemose class. 

 As examples, it will be sufficient to mention the salivary, lacrymal, and mammary 

 glands. 



Of the tubular compound glands, the most characteristic examples are the 

 testicle and kidney. In these the tubular ducts divide again and again into branches, 

 which, retaining their tubular form, are greatly lengthened out The branches of 

 the ducts are formed, as usual, of a limitary or basement-membrane (membrana 1 



