HISTOLOGY OF THE FROG 83 



motion altogether. The cilia pass into a stiffened condition, 

 known as heat rigor, from which they do jiot recover. In 

 warm-blooded animals the optimum temperature for ciliary 

 motion is about 45 deg. C. In cold-blooded animals the tempera- 

 ture of the body appears to be the optimum, but as this varies 

 according to the temperature of the surrounding medium, we 

 find the cilia more active in summer heat than in winter cold. 

 Oxygen, at least in its free state, is not essential to the action 

 of cilia, as may be shown by the fact that it will go on for 

 some time in water, which has been deprived of all its dis- 

 solved oxygen by boiling. This experiment, however, does not 

 prove that the source of energy is anything else than the oxida- 

 tion of the substance of the ciliated cells, but rather that the cells 

 can store up oxygen in a combined form for use when required. 

 Carbonic acid gas has a definite effect in arresting ciliary motion, 

 as can be seen if a stream of the gas is passed over a prepara- 

 tion of active ciliated cells. Their motion is at first accelerated, 

 but soons slows down, and may be arrested altogether. It 

 soon is restored, however, on the readmission of fresh air. 

 Chloroform vapour acts in the same way as carbonic acid gas. 

 Ciliated cells are found in various parts of the frog's body : 

 in the pharynx and olfactory passages; on the edges of the 

 partitions separating the alveoli of the lungs ; in the central 

 canal of the spinal cord, and the ventricles of the brain ; in the 

 uriniferous tubules of the kidney, and in the ureters ; in the 

 oviducts, and, in the tadpole, on the epithelium covering the 

 transitory external gills. 



Both columnar and ciliated epithelia belong to the variety 

 of epithelium known as simple, because their component cells 

 are arranged in a single layer. Another variety of simple 

 epithelium is that forming the thin membranes lining internal 

 cavities, such as the ccelom, the heart and blood-vessels, the 

 lymph-spaces, etc. This variety is often referred to under the 

 name endothelium, but may more conveniently be described 

 as pavement epithelium. Pavement epithelium may be very 

 advantageously studied in the mesentery of the frog. Both 

 sides of the mesentery are covered with a very thin membrane 

 continuous with that lining the pleuro-peritoneal cavity. This 

 membrane is composed of very thin, flat, scale-like cells, which 

 may be polygonal, and fit together by their edges, like the tiles 

 in a pavement, but more usually have serrated or sinuous 



