

SIMPLE ISOLATED CELLS. EPITHELIUM. 147 



in propelling the various products of secretion. The case is different, 

 however, among animals of the lower classes, especially those inhabiting 

 the water. Thus the external surface of the gills of Fishes, Tadpoles, 

 &c., is furnished with cilia ; the continual movement of which renews 

 the water in contact with them, and thus promotes the aeration of the 

 blood. In the lower Mollusca, and in many Zoophytes, which pass their 

 lives rooted to one spot, the motion of the cilia serves not merely to 

 produce currents for respiration, but likewise to draw into the mouth the 

 minute particles that serve as food. And in the free-moving Animal- 

 cules, of various kinds, the cilia are the sole instruments which they 

 possess, not merely for producing those currents in the water which may 

 bring them the requisite supply of air and food, but also for propelling 

 their own bodies through the water. This is the case, too, with many 

 larger animals of the class Acalephae (Jelly-fish), which move through 

 the water, sometimes with great activity, by the combined action of the 

 vast numbers of cilia that clothe the margins of their external surfaces* 

 In these latter cases it would seem as if the ciliary movement were more 

 under the control of the will of the animal, than it. is where it is con- 

 cerned only in the organic functions. In what way the will can influence 

 it, however, it does not seem easy to say ; since the ciliated epithelium- 

 cells appear to be perfectly disconnected from the surface on which they 

 lie, and cannot, therefore, receive any direct influence from their nerves. 

 Of the cause of the movement of the cilia themselves, no account can be 

 given; they are usually far too small to contain even the minutest 

 fibrillse of muscle ; and we must regard them as being, like those fibrillse, 

 organs sui generis, having their own peculiar endowment, which is, in 

 the higher animals at least, that of continuing in ceaseless vibration, 

 during the whole term of the life of the cells to which they are attached, 

 The length of time during which the ciliary movement continues after 

 the general death of the body, is much less in the warm-blooded than 

 in the cold-blooded animals ; and in this respect it corresponds with 

 the degree of persistence of muscular irritability, and of other vital 

 endowments. 



236. The Tessellated-Epithelium, as already mentioned, covers the 

 Serous and Synovial membranes, the lining membranes of the blood- 

 vessels and absorbents, and the Mucous membranes with their glandular 

 prolongations, except where the cylinder-epithelium exists. It presents 

 itself, with some modifications presently to be noticed, in the ultimate 

 follicles of all glands, and also in the smaller bronchial tubes. In this 

 latter situation it is furnished with cilia ; and these are also found on 

 the cells of the tessellated epithelium, which covers the delicate pia mater 

 lining the cerebral cavities. The Cylinder-Epithelium commences at 

 the cardiac orifice of the stomach, and lines the whole intestinal tube ; 

 and, generally speaking, it lines the larger gland-ducts, giving place to 

 the tessellated form in their smaller ramifications. A similar epithelium, 

 furnished with cilia, is found lining the air-passages and their various 

 offsets, the nasal cavities, frontal sinuses, maxillary antra, lachrymal 

 ducts and sac, the posterior surface of the pendulous velum of the palate 

 and fauces, the eustachian tubes, the larynx, trachea, and bronchi, 

 becoming continuous, however, in the finer divisions of the latter, with 



