396 OF SECRETION. 



which are the real agents in the secreting process ( 239). Why one 

 set of cells should secrete Bile, another Urea, and so on, we do not 

 know ; but we are equally ignorant of the reason, for which one set 

 of cells converts itself into Bone, another into Muscle, and so on. 

 This variety in the endowments of the cells, by whose aggregation 

 and conversion the fabric of the higher Animals is made up, is a fact 

 which we cannot explain, and which must be regarded (for the present, 

 at least,) as one of the "ultimate facts" of Physiological Science. 



716. Passing by the extended membranous surfaces, and the protec- 

 tive cells with which they are covered, we find that the simplest form 

 of a secreting organ is composed of an inversion of that surface into a 

 series of follicles, which discharge their contents upon it by separate 

 orifices. Of this we have an example in the gastric follicles, even in 

 the higher animals ; the apparatus for the secretion of the Gastric fluid 

 never attaining any higher condition, than that of a series of distinct 

 follicles, lodged in the walls of the stomach, and pouring their products 

 into its cavity by separate apertures. In Fig. 107 is represented a 

 portion of the Ventriculus succenturiatus of a Falcon; in which the 

 simplest form of such follicles is seen. A somewhat more complex 

 condition is seen in some of the Gastric follicles of the Human stomach 

 (Fig. 80) ; the surface of each follicle being further extended by a sort 

 of doubling upon itself, so as to form the commencement of secondary 

 follicles, which open out of the cavity of the primary one. Now a con- 

 dition of this kind is common to all glands, in an early stage of their 

 evolution ; and in this stage, we meet with them, either by examining 

 them in the lowest animals in which they present themselves, or by 

 looking to an early period of their embryonic development in the 

 highest. Thus, for example, the Liver consists, in certain Polypes and 

 in the lowest Mollusca, of a series of isolated follicles, lodged in the 

 walls of the stomach, and pouring their product into its cavity by sepa- 

 rate orifices ; these follicles being recognised as constituting a biliary 

 apparatus, by the colour of their secretion. And in the Chick, at an 



Fig. 107. Fig. 108. 



Glandular follicles in ventriculus Origin of the Liver from the intestinal wall, in 



euccenturiatus of Falcon. the embryo of the Fowl, on the fourth day of in- 



cubation: a, heart; b, intestine; c, everted por- 

 tion giving origin to liver; d, liver; e, portion of 

 yolk-bag. 



early period of incubation, the condition of the Liver is essentially the 

 same with the preceding ; for it consists of a cluster of isolated follicles, 

 not lodged in the walls of the intestine, but clustered round a sort of 



