From Treatise on human physiology by Henry Cadwalader Chapman, Published by Lea Brothers & Co, 1887, page 446.

“A more modern apparatus used by Ludwig and his pupils, differs from that of Regnualt and Reiset just described, not so much in principle as in certain mechanical details. The most noticeable of these is the ingenious contrivance by means of which the oxygen expired passes from g (Fig. 251) into the respiratory tube d, communicating through an air-tight covering with the nostrils of the animal at e, alternately with the passage of the carbonic acid expired into the bulbs f, and which is accomplished through the alternate expansion and contraction of valve c. For with the rarefaction of the air through inspiration the valve c is drawn from the end of the tube b, the effect of which is that the air entering the tube b drives the water out of a, which in turn drives the oxygen out of g into the tube d.  On the other hand, with the condensation of the air through expiration, the valve c is forced back close to the end of tube b, the flow of oxygen from the tube d ceases, the carbonic acid exhaled passing into the bulbs f.”