STOP OR CLICK MECHANISM IN THE ANIMAL KING- 

 DOM. By Otto Thilo, M.D., Riga. (Plates XXV., XXVI.) 



Stop mechanisms are found very often in the animal kingdom. 

 They appear in a variety of forms and consist of different 

 substances, but serve always the same purpose, viz., the econo- 

 mising of labour. 



Always where it is necessary to hold a part of the body 

 steadily in the same position for a length of time, one finds the 

 strain on the muscles taken off by a click mechanism. 



When, for example, the passage of liquid through an elastic 

 tube or pipe is to be prevented by the strength of the muscles, 

 it would be done by holding the pipe firmly together, as one 

 would bind a cord round an india-rubber hose. The muscles 

 would, of course, soon tire ; consequently, in such cases, one finds, 

 in place of a muscle contraction, an automatic lock, that is, a 

 stop or click mechanism. The best example of this is found 

 in the valves of the heart. These develop out of the creases of 

 the lining of the blood-vessels ; the creases increase, and by 

 degrees take a pocket form, thus becoming valves. Valves 

 resembling these which prevent the back flow of the blood are 

 also to be found in the veins. The veins have much thinner 

 walls than the arteries. The arteries have no valves, and the 

 circulation of the blood is regulated only by the strength of the 

 muscular coat. 



The heart valves admit only of the flow of blood in one 

 direction, therefore we may call them running ^ catch mechanisms 

 for fluids. One finds in many animals also running catch 

 mechanisms for gaseous bodies, as, for instance, on the strange 

 globular fishes which blow themselves up like a balloon. 



A third group of catch mechanisms is to be found in the 



^ Under running catch mechanism is understood a stop mechanism which 

 prevents motion in one direction, but admits of it in the other, as, for 

 example, the mechanism of a ship's anclior windlass. Stop mechanisms which 

 prevent motion in both directions are called stationary. Vide Reuleaux, Kron- 

 strudeur, Braunschweig, 1895, page 600. 



