360 MANUAL OF HISTOLOGY. 



first as constituents of the gland cells, \ve find, nevertheless, that they 

 differ among themselves in two particulars. 



In the first place we remark, that in a certain number of the organs in 

 question these substances are only abstracted from the blood to sojourn 

 simply in the body of the cell for a longer or shorter space of time. This 

 is the case, for instance, with the constituents of the sweat glands and 

 kidneys, in which we are unable to demonstrate any notable chemical 

 metamorphosis through the agency of the cell. The latter may, however, 

 be evident, though in a minor degree, in other glands, for instance in the 

 female breast, in which an albuminous substance is transformed into 

 casein, and, we suppose, grape sugar into sugar of milk. Such instances 

 are connecting links between the first case and another, in which the cell 

 produces, by the disintegration and rearrangement of the matters it 

 receives, completely new and peculiar substances, as may be seen in the 

 liver, in the production of the bile acids. 



Another difference concerns the cell itself, as we know already. This 

 may either be cast off after the generation of its specific contents, setting 

 free the latter (sebaceous, milk, and peptic cells), or the contents may 

 escape from its uninjured body, while it itself remains as a permanent 

 structure (renal and hepatic cells). 



Finally, the "egotistical" mutation of matter of glandular tissue, i.e., 

 that which takes place in the interest of its own proper nutrition, must 

 give rise to the generation of the more general decomposition products 

 of the system. Thus, according to Staedeler and Frerichs, leucin has 

 been found in exceedingly small quantity as a very general transforma- 

 tion product in glands, seldom in larger amount, as in the pancreas. 

 Other bases, such as tyrosin, taurin, cystin, hypoxanthin, xanthin, and 

 guanin, appear more rarely. Iriosit and lactic acid may also be met with, 

 and uric acid, though with less frequency. These matters are partly dis- 

 charged with the secretions of the glands, and partly taken up again into 

 the circulation. 



Later on, in considering the salivary glands, we shall see the control 

 which the nervous system possesses over the chemical action of these organs 



200. 



Turning now to the development of glands, it will be remembered that 

 the epithelial nature of these structures has already been touched on. 

 The mode of origin is the best proof of this. It is well known that a 

 whole series of glandular organs is derived from the external cellular 

 layer of the foetal body from the so-called corneous leaf. They commence 

 in the form of nodulated prolongations downwards of the epithelial cells, 

 ill which at first no trace exists of either central cavity or gland-mem- 

 brane. This latter is subsequently formed on the exterior of the aggrega- 

 tion of cells as a deposit. The size of this mass is increased by division of 

 the cells of which it is composed, while the connective-tissue surrounding 

 it becomes eventually the envelope of the gland. Among the structures 

 so formed may be mentioned the sweat, mammary, and lachrymal glands. 



The sweat glands (fig. 353, a) are developed, according to Koelliker, 

 after the fifth month of intra-uterine life. Commencing as small flask- 

 shaped growths formed of the rete Malpujldi cells, .they advance deeper 

 downwards through the skin in the following months, becoming eventu- 

 ally curved, in a gradual manner, at their lower end. Then a trace of 

 the central passage and external outlet becomes apparent. The sebaceous 



