26 



THE CAT. 



[chap. II. 



Thus tlie real body of the animal lies enclosed between the external 

 skin and its internal reflected continuation, and answers to the en- 

 closed interior of the ring-like air-cushion. A real " body cavity " is 

 therefoi'c not the inside of the alimentary canal, or the inside of any 

 other passage opening on the exterior ; all such passages being of 

 course but so many continuations inwards of external space. A real 

 "body cavity" would be any cavity existing enclosed between the 

 external skin and its internal reflected continuation. This reflected 

 skin is soft and delicate, with a moistened surface, and is called 

 " mucous membrane.'" Such membrane lines two great sets of 

 organs. One of these is the gastro-piiliiionari/ mucous membrane, 

 and Imes the mouth and alimentary canal, the eyelids, ears, nostrils, 

 cavities in the skull, and the windpipe and lungs. The other is 

 called the rjenito-urinary, and lines the bladder and the parts con- 

 nected with its passage outwards. 



§ 15. Just as the external skin consists of epidermis and dermis, so 

 its reflected portion consists of a non- vascular ejjif/ieliuni, with a sub- 

 jacent highly vascular coriuni, which often contains much muscular 

 fibre. Between them is the homogeneous structureless layer termed 

 the basement membrane. The component cells of the epithelium 

 may be elongated at right angles to the basement membrane, thus 

 forming what is called " cohiuinar epifheliitm " (as in the stomach and 

 intestine), or they may be rounded, forming spheroidal ejrif helium, as 

 in the lining of the ducts of the " glands " * of the alimentary canal. 



Sometimes parts of the substance of epithelial cells may protrude 

 as thread-like processes or cilia, which arc capable of performing 

 repeatedly a whipping-like movement. A membrane consisting of 

 such cells is called ciliated epithelium (Fig. 11), and such we shall find 

 in certain of the cat's alimentary and respiratory organs, in the 

 description of which f this kind of tissue will be again noticed. 



The coriuii) contains yellow (or elastic) as well as white fibres, 

 and the supjily of either may be copious or scanty. Its surface may 

 be even or very uneven. Thus it may be produced into many, 

 often relatively large, papilla) or villi — scattered or closely set — or 

 into ridges which may so intersect as to form polygonal pits between 

 them. 



Just as the outer skin is furnished with sweat and sebaceous 

 glands, so also mucous membrane is copiously furnished with small 

 glands which have different functions'tin different parts ; but a generally 

 diffused secretion, called mucus, is formed by them, which gives its 

 name to the membrane in which its formative glands are imbedded. 

 It is slightly alkaline, and serves to preserve the moisture of the 

 surfaces it lubricates, as well as to protect them from the dissolving 

 action of fluids secreted to dissolve and digest food temporarily held 

 within cavities (the stomach, &c.) which are lined by mucous 

 membrane. 



• Epitlielial cells may, as wc slial 

 hereafter sec, take on the function 

 manufacturing some special product 



"secretion." Parts which thus act are 

 termed " glands." 



t See below, Chapter VI. 



