SALIVARY GLANDS. 



427 



which only one gland was present, running at 

 right angles to the middle line. It was convex 

 in front and concave behind, having a trans- 

 verse diameter of one-third of an inch, an 

 antero-posterior one-eighth of an inch. It 

 gave off three delicate ducts. 



The minute structure of the glands in general 

 has been already fully inquired into *, and to 

 the type on which they are formed the salivary 

 glands offer no exception. A simple caeca! 

 membranous prolongation is the model to 

 which they can all be referred, however complex 

 each individual series of glands may appear. 

 This grand generalisation, by which the ex- 

 treme simplicity of the operations of nature is 

 remarkably illustrated, has mainly been the 

 result of a minute inquiry into developmental 

 and comparative anatomy. We are particu- 

 larly indebted, however, to Miiller and E. H. 

 Weber for the exposition of the evolution 

 and minute structure of the salivary glands. 



Miiller thus describes the first appearance 

 of a salivary gland in Mammalia, and his ob- 

 servations were taken from the embryo of a 

 sheep, two inches long : Its form is that of 

 a simple canal wirh bud-like processes, lying 

 in a gelatinous nidus or blastema, and com- 

 municating with the cavity of the mouth. As 

 the development of the gland advances, the 

 canal becomes more and more ramified, in- 

 creasing at the expense of the germinal mass 

 or " blastema," in which it is still enclosed. 

 The blastema soon acquires a lobulated form, 

 corresponding to that of the future gland, and 

 is at last wholly absorbed. Valentint remarks 

 that a portion of this blastema, which contains 

 nuclei and cell-formations, and which is not 

 converted into glandular structure, is changed 

 into blood-vessels, nerves, and connecting 

 cellular tissue; and he has, further, accu- 

 rately determined that the secondary tubes 

 are formed independent of the primary, at the 

 expense of portions of the blastema, in the 

 vicinity of the main duct, with which, by a 

 centripetal development, they ultimately com- 

 municate. Thus, in the first stage of their 

 development, the salivary ducts can be seen 

 to constitute an independent closed system of 

 tubes. The investigations of E. Weber j 

 carry us a step further in the inquiry. He 

 found, by a successful injection of the parotid 

 in a human foetus, that the excretory duct, 

 after having undergone its ultimate state of 

 subdivision, by an extensive ramification of its 

 secondary tubes, terminated in microscopic 

 twigs, each twig having appended to it one or 

 more minute cells or vesicles, forming small 

 group-like lobules or bunches. These cells 

 have not a uniform size, their long diameter, 

 which is more or less in a line with the axis 

 of each of the terminal divisions of the duct 

 with which the cells are structurally conti- 

 nuous, is, on the average, almost -^ of a 



Paris line. Gerber * states these vesicles or 

 cells are variously shaped, from ^ to T i_ 

 of a Paris line in diameter, and "upon the 

 periphery of the gland appear mutually to 

 compress each other and to become poly- 

 hedral in their outline. They are united to- 

 gether into small lobules, from four to seven 

 times greater than each individual vesicle, the 

 latter consequently being almost three times, 

 the former about twelve times the diameter 

 of the capillary blood-vessels which ramify on 

 the surface. They form, in fact, the caecal 

 terminations of the branches of the excretory 

 tubes, without having of necessity an indi- 

 vidual narrow connecting pedicle, as figured 

 by Berres-j- in the minute anatomy of the 

 parotid. 



Such, then, is the essential structure of the 

 salivary glands ; and in the full state of or- 

 ganisation of each we recognise the elements 

 of a mucous membrane, constituting the in- 

 ternal lining of the excretory duct and conti- 

 nuing throughout the series of its ultimate 

 ramifications as far as the terminal vesicles ; 

 a middle elastic coat, and an external covering 

 of areolar tissue. The mucous membrane 

 consists of an epithelial layer, and a basement 

 membrane. The epithelium is of the co- 

 lumnar variety, and maintains this character 

 along the track of the excretory duct as far 

 as its delicate divisions, where it gradually 

 changes its character, so that that lining the 

 interior of the vesicles is of the pavement type. 

 This transition of columnar into pavement 

 epithelium would appear gradual; so that it 

 is difficult to determine the point at which the 

 one form terminates and the other commences. 

 The basement membrane is continued along 

 the entire track of the tubular ramifications, 

 as far as the vesicles, the form of which it 

 would appear to determine. There can be 

 little doubt that this is the membrane which 

 Berres alludes to as the proper wall of the 

 vesicles, and describes as a small transparent 

 membrane, covered over with molecules, and 

 which also has been represented by Henle as 

 homogeneous, but which he at the same time 

 considers as composed of filaments of cellular 

 tissue solidly united together.^ 



Considerable difference of opinion has ex- 

 isted as to the nature of the middle coat of the 

 glandular tubes, according as the largest or 

 smallest have been examined. Valentin re- 

 marks that in the first case it has been considered 

 fibrous, in the second simply homogeneous. In 

 by far the greater number of the terminal extre- 

 mities of the glandular tubes the intermediate 

 membrane appears clear and transparent, and 

 gives neither in the fresh state, nor when re- 

 agents are applied, any indication of a fibrous 

 character. In all the large tubes the intermediate 

 coat is formed of distinct flat fibres, together 

 with the characteristic fibres of cellular tissue, 



* Vide Article GLAND. 



f Wagner's Handworterbuch der Physiologic, 

 Article GEWEBE. 



J Meckel's Archiv. fur Anatomic et Physiologic, 

 1827. 



* Gerber, General and Minute Anatomy of Man 

 and the Mammalia, translated by E. Gulliver, 1842. 



f Anatomia Microscopica Corporis Humani, tab. 

 ix. fig. 2. 



I Mullet's Archiv. 1838, p. 105. 



