SECRETION. 247 



ing the secreted fluid to a more distant point. These cells,- 

 or follicles, as they are termed, are generally employed for 

 the mucous secretions, and are often scattered throughout 

 the surfaces of membranes:* at other times the secreting 

 cavities are collected in great numbers into groups; and 

 they then frequently consist of a series of lengthened tubes, 

 like caeca, examples of which we have already seen in the 

 hepatic and salivary glands of insects. 



A secretory organ, in its simplest form, consists of short, 

 narrow and undivided tubes; we next find tubes which are 

 elongated, tortuous or convoluted, occasionally presenting di- 

 lated portions, or even having altogether the appearance of a 

 collection of pouches, or sacs; while, in other cases, they are 

 branched, and extend into minute ramifications. Sometimes 

 they are detached, or isolated; at other times they are collected 

 into tufts, or variously grouped into masses, where still the se- 

 parate tubes admit of being unravelled. The secreting fila- 

 ments of insects float in the general cavity, containing the 

 mass of nutrient fluid, and thence imbibe the materials they 

 require for the performance of their functions. It is only 

 when they receive a firm investment of cellular membrane;, 

 forming what is termed diCapsule, and assuming the appear- 

 ance of a compact body, that they properly constitute a 

 gland; and this form of a secreting organ is met with only 

 among the higher animals.t 



Great variety is observable both in the form and struc- 

 ture of difl'erent glands, and in the mode in which their 

 blood vessels are distributed. In animals which are fur- 

 nished with an extensive circulation, the vessels supplying 

 the glands with blood are distributed in various modes; and 

 it is evident that each plan has been designedly selected 

 with reference to the nature of the particular secretion to 



* See p. 135 of this volume; and in particular Fig. 305. Sebaceous folli- 

 cles are also noticed in Vol. i. p. 91. 



•j- Dr. Kidd, however, describes bodies apparently of a glandular charac- 

 ter, disposed in rows on the inner surface of the intestinal canal of the Gryl' 

 lotalpa, or mole-cricket. Phil. Tran. for 1825, p. 227. 



