TISSUES OF THE BOD Y. 



345 



In the earlier days of anatomical study a round form, soft consistence, 

 and great vascularity, sufficed to gain for an organ the name of " gland." 

 Later on, however, the physiological requisites for the proper conception 

 of a gland became more prominent. And first of all, that the latter 

 abstract from the blood matters which are not to be made use of for its 

 own nutrition, but which tend to benefit the whole system, either by 

 being cast out of the body as decomposed material to be gotten rid of, or 

 turned to account in the economy as specially prepared by the gland. 

 Thus, the latter came to be looked upon as a secreting organ, great stress 

 being laid, consequently, on its efferent duct. Finally, it was recognised 

 that there are many completely closed organs from which no secretion is 

 ever given off, and to which, nevertheless, we cannot deny the right to be 

 called glandular structures. This was subsequent to comparative anatomy 

 having shown the comparatively small 

 weight to be given to the duct as the 

 distinguishing mark of glands. 



Recent microscopical analysis has 

 supplied us with characteristic signs by 

 which, in general, a gland may be diag- 

 nosed, although there remain certain 

 points relating to structure about which 

 doubt still exists. 



The history of development like- 

 wise has also afforded most important 

 information here. From it we learn 

 that the physiologically important parts 

 of true glands, namely, their secreting 

 cells, all take origin either from the 

 corneous or intestino-glandular em- 

 bryonic leaves. JS"o truly glandular organs spring from the middle 

 germinal plate. 



Finally, owing to our extended knowledge of the nature of the lym- 

 phatic apparatus, we are now 

 enabled to class with the latter 

 as lymphoid organs a series 

 of parts springing from the 

 middle embryonic leaf, which 

 used formerly to be reckoned 

 among the glands. 



Let us now return to the 

 histogical characters of glands. 

 These organs consist of two 

 kinds of structural elements 

 (figs. 325 and 326) (1) of a fine 

 structureless transparent mem- 

 brane, known as the membrana 

 propria, which determines the 

 form of the organ as well as 

 that of its sub-divisions, and 

 (2) of the contents of the 

 latter, the so-called gland cells 



(figs. 325, 327. and 328). As a third indispensable factor, we find on 

 the external surface of the homogeneous membrane a vascular network (fig. 



Fig. 327. Gastric glands from the dog, filled 

 with cells, and interlaced by a vascular net- 

 work. 



Fig. 328. Lobule from the liver of a boy ten years old. 



