ESSENTIAL STRUCTURE OF SECRETING GLANDS. 



301 



Crustacea, by referring to fig. 47, fo ; and in the Mollusca it is 

 nearly the same (figs. 157, /, and 149,/). 



357. The required extent of secreting surface is not unfre- 

 quently given, however, by the prolongation of the follicles 

 into tubes, rather than by a great multiplication in their 

 number. Of this we have a remarkable example in the 

 Kidney of the higher animals ( 368), which is entirely com- 

 posed of such tubes, together with areolar tissue which binds 

 them together, and the blood-vessels distributed amongst them. 

 These tubes, like the follicles, are lined with epithelium-cells 

 (fig. 166), which are the real instruments in the separation of 

 their secreted product. 



358. That there is nothing in the form of any secreting 

 apparatus, however, which determines the peculiar nature of 

 its secretion, is evident from this 



fact, that, in glancing through the 

 Animal series, we find the same secre- 

 tion elaborated by glandular struc- 

 tures of every variety of form. Thus, 

 we have seen that the bile is secreted, 

 in the lowest animals in which we 

 can distinguish it, by a number of 

 distinct follicles, as simple in their 

 structure as are those by which the 

 mucous secretions are formed in the 

 highest. Again, the bile is secreted 

 in Insects, by a small number of long 

 tubes, which open separately into the 

 intestinal canal j ust below the stomach 

 (fig. 112); and these tubes appa- 

 rently differ in no respect from those 

 that form the urinary secretion in the 

 same animals, which open nearer the 

 outlet of the intestinal canal. In 

 fact, the distinct function of the 

 latter was not known, until it was 

 ascertained that uric acid is to be 

 found in them. In fig. 167, which 

 represents the digestive apparatus of the Cockchafer, it is 

 seen that the biliary vessels are only four in number, but 

 are very long ; and that, for a good part of their length, 



Fig. 167. 



ALIMENTARY CANAL AND 

 HEPATIC TUBULES OF COCK- 



CHAFER. 



