240 



OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



of the digestive process, become turgid with cells, that accumulate in such 

 quantity, as to give to the tubes a sacculated appearance which they do not possess 

 when empty; within the principal cells, smaller ones are frequently observable, 

 and even a second brood may be sometimes seen in the interior of the latter; 1 

 and when digestion is going on, these cells are poured out in large numbers on 

 the surface of the mucous membrane, where they undergo a kind of deliques- 

 cence by the imbibition of water, and form the substance, indefinitely termed 

 mucus, which probably contains the " ferment" that is the essential accompani- 

 ment of the acid solvent in the process of gastric digestion. Besides these folli- 

 cles, however, which are by no means peculiar to the lining of the alimentary 

 canal, the gastro-intestinal mucous membrane contains numerous other simple 

 glandulse, which afford links of transition towards those more complicated forms 

 of the glandular apparatus that are less intimately connected with it. Reserv- 

 ing a more particular description of these for a future opportunity (CHAPS, vii. 

 and xni.), we shall here only notice the points that bear upon the essential 

 nature of Glands in general. Various parts of the mucous membrane of the 

 stomach, and of the large intestine, are studded at intervals with shallow pits or 

 follicles, about l-20th of an inch in diameter; which, according to the observa- 

 tions of Dr. A. Thomson (loc. cit.), have the form of closed vesicles during foetal 

 life and early infancy, but gradually open, so that their cavities become continu- 

 ous with the free surface of the mucous membrane, the columnar epithelium of 

 which extends itself into them ; and they remain in that condition during the 

 rest of life. What their distinctive attribute may be, however, has not been 

 made out. The mucous lining of the small intestine is beset at intervals with 

 elevated patches, which are known under the name of the " agminated glands of 

 Peyer." These are formed by the aggregation of originally-closed vesicles of a 

 somewhat lenticular shape, which lie just beneath the mucous membrane, their 

 own walls being closely incorporated, at the deeper side, with the subjacent fila- 

 mentous tissue; and they are filled with cells and granular particles in various 

 stages of development. It seems probable from the observations of Profs. Krause 

 and Allen Thomson, that these vesicles are continually opening and discharging 

 their contents upon the mucous surface; each, when it has completely emptied 

 itself, becoming atrophied, and being replaced by another. These, also, are appa- 

 rently to be looked upon as secreting organs ; but they can scarcely be considered 



in any other light than as parent-cells, 



Fig. 29. 



Portion of one of Srunner's Glands, from the Human 

 Duodenum. Magnified 65 diameters. 



developed in the substance of the tis- 

 sues quite independently of the mucous 

 surface, with which they only become 

 connected for the purpose of giving exit 

 to their contents. Such, it is probable, 

 is the original state of the elements of 

 most of the more complex glandular 

 structures ; the essential part of them 

 seeming to consist of a collection of 

 glandular vesicles, which are originally 

 closed (a condition that is retained in 

 the thyroid throughout life), but which 

 afterwards come into connection with 

 the mucous or cutaneous surface where- 

 on they discharge their secretion, by 

 an extension of an offset from the lat- 

 ter, that constitutes the ramifying 

 ducts on which they open. A simple 



1 Dr. Allen Thomson, in Goodsir's "Annals of Anatomy and Physiology," No. I. p. 36. 



