126 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



are endowed in a remarkable degree with absorbing power, whilst they 

 are also furnished with numerous glandulse,, which pour the solvent fluid 

 into the cavity. On the other hand, it has an outlet, through which the 

 indigestible residuum is cast forth, together with the excretions from the 

 various glands that pour their products into the alimentary tube. In the 

 bronchio-pulmonary apparatus, the same outlet serves for the introduc- 

 tion and for the expulsion of the air ; and here, too, is continual change. 

 In other cases, there is but a single outlet ; and the change is of a simpler 

 character, consisting merely in the expulsion of the matters eliminated 

 from the blood by the agency of the glands. Now it is, as we shall see 

 hereafter, in, the digestion and absorption of food, on the one hand, and 

 in the rejection of effete matters on the other, that the commencement 

 and termination of the nutrient processes consist ; and these operations 

 are performed by the system of Mucous membranes, including in that 

 general term the Skin, which is an important organ of excretion, besides- 

 serving as the medium through which sensory impressions of a general 

 character are received by the Nervous system. 



204. The Mucous Membrane may be said, like the Serous, to consist 

 of three chief parts ; the epithelium or epidermis covering its free sur- 

 face ; the subjacent basement-membrane ; and the areolar .tissue, with 

 its vessels, nerves, &c., which forms the thickness of the membrane, and 

 connects it with the subjacent parts. The Epidermis and Epithelium 

 alike consist of cells ; but the function of the former (which consists of 

 several layers, of which the outer are dry and horny) is simply protec- 

 tion to the delicate organs beneath ; whilst that of the latter is essen- 

 tially connected with the process of Secretion, as will be shown hereafter. 

 The basement-membrane resembles that of the serous membranes ; but 

 its separate existence is unusually evident in some parts where it exists 

 alone, as in the tubuli uriniferi of the kidney ; whilst it can with diffi- 

 culty be demonstrated in others, as the skin. The Areolar tissue of 

 Mucous membranes usually makes up the greatest part of their thick- 

 ness ; and it is so distinct from that of the layers beneath, constituting 

 the sub-mucous tissue, as to be readily separable from them. It differs 

 not in any important particular, however, from the same tissue else- 

 where ; and the white and fibrous elements may be detected in it in 

 varying proportions, in different parts, the latter being especially 

 abundant in the skin and lungs, which owe to it their peculiar elas- 

 ticity. Hence the Mucous membranes yield Gelatine in abundance, on 

 being boiled. The skin also appears to contain some of the non-striated 

 Muscular fibre ( 337), in varying proportions in its different parts. 



205. The relative amount of Blood-vessels, Nerves, and Lymphatics, 

 as already mentioned, is subject to great variation, according to the 

 part of the system examined. The first, however, are most constantly 

 abundant, being required in the Skin for sensation (Fig. 9), and in the 

 Mucous membranes for absorption and secretion (Figs. 10, 11, 12). In 

 fact we might say of many of the mucous membranes, especially those 

 of the glands, that their whole purpose is to give support to the secreting 

 cells, and to convey blood-vessels into their immediate neighbourhood, 

 whence these cells may obtain materials for their development. The 

 Skin is the only part of the whole system which is largely supplied with 



