148 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



the ciliated pavement-epithelium. The upper part of the vagina, the 

 uterus, and the fallopian tubes, are also furnished with a ciliated Cylin- 

 der-Epithelium. The function of the cilia in all these cases appears to 

 be the same ; that of propelling the viscid secretions, which would 

 otherwise accumulate on these_ membranes, towards the exterior orifices, 

 whence they may be carried off. 



237. The simplest office which the Epithelium-cells of Mucous mem- 

 branes perform, appears to be that of elaborating a peculiar secretion 

 termed Mucus ; which is destined to protect them from the contact of 

 air, or from that of the various irritating substances to which they are 

 exposed, in consequence of their peculiar position and functions. This 

 Mucus is a transparent semifluid substance, distinguished by its peculiar 

 tenacity or viscidity. It is quite insoluble in water ; but is readily dis- 

 solved by dilute alkaline solutions, from which it is precipitated again by 

 the addition of an acid. A substance resembling Mucus may be pro- 

 duced from any fibrinous exudation, or even from pus, by treating it 

 with a small quantity of liquor potassse. The secretion of Mucus, like 

 the formation of Epidermis, appears to take place with an activity pro- 

 portioned to the degree of irritation of the subjacent membrane. On 

 many parts of the mucous surface, a sufficient supply is afforded by the 

 epithelium-cells which cover it ; but in other situations, especially along 

 the alimentary canal, the demand is much greater, and it is probably 

 supplied not merely by the cells of the surface, but by those lining the 

 crypts or follicles which are formed by involutions of it. 



238. The Epithelium-cells, which are thus being continually renewed 

 on the Mucous surfaces, commonly seem to have their origin in the 

 granular germs diffused through the basement-membrane ; but it is dif- 

 ferent in regard to the cells of the follicles, which seem rather to occupy 

 their cavity than merely to line their walls, and which appear to be in 

 course of continual production from a germinal spot, or collection of re- 

 productive granules, at the blind extremity of the follicle. This is the 

 case in the ultimate follicles of the more complex glands ; which may be 

 regarded as so many repetitions of the simple crypts or follicles in the 

 substance of the mucous membranes ; the only difference being, that 



Fig. 27. 



Fig. 28. 



Fig. 29. 



Two follicles from the liver of Carcinus 

 manas (Common. Crab), with their con- 

 tained secreting cells. 



Ultimate follicles of Mammary 

 gland, with their secreting cells, 

 a, a; b, b, the nuclei. 



Secreting cells 

 Human Liver; a, 

 cleus; b, nucleolus; 

 oil-particles. 



the former pour their secretion into a branch of a duct, which unites with 

 the other ramifications to form a trunk ; and this trunk conveys them to 

 their destination in some cavity lined by a mucous membrane ; whilst 

 the simple follicles or crypts at once pour forth their secretion upon the 



