534 PRINCIPLES OF GENERAL PHYSIOLOGY 



Maewilliam (1902) found that excised mammalian arteries pa-- readily into 

 a state of contraction, which seems to depend upon a supply of oxygen. They 

 can be relaxed by carbon dioxide. Similar observations on the effect of oxviri-n 

 and carbon dioxide were made by Severini (1878, p. 93, and 1881) on the 

 mesenteric vessels of the frog. 



Reaction to Stretching. The denervated smooth muscle of the earthworm was 

 shown by Straub (1900) to respond to stretching by a contraction (see Fig. 

 132, page 436). The stomach of the frog behaves similarly (Winkler, 1898). 

 The question of the behaviour of the arterioles will be discussed in Chapter XXIII. 

 The Muscles of the Chromatophores of the Cephalopod. Hofmann (1907, 3) 

 showed that there are no peripheral ganglia in this case, but that the ton us 

 returns after section of the nerves. He is inclined to attribute it to an cfl'cct <f 

 carbon dioxide in moderate concentration. 



The Adductor Muscle of Anodonta. Pavlov (1885, pp. 21, 22) showed that 

 the tonus of this muscle is not due to nervous impulses from ganglion cells, simv 

 it does not disappear when the visceral ganglion is removed, and there art- no 

 ganglion cells in the muscle itself. But stimulation of the nerves from tin- 

 visceral ganglion to the muscle, after the ganglion itself has been cut out, cau-r- 



inhibition. 



This reference to tlu> 

 adductor muscle of the 

 Mollusc leads us to consider 

 some noteworthy peculiarities, 

 which are most easily inves- 

 Fic;. 169. DIAGRAM TO ILLUSTRATE A CATCH OK RATCHET tigated in these organisms, 

 MECHANISM. The upper piece can be pushed in the although they set-in to be 

 direction of the arrow and the total length of the niore or less present in all 

 model shortened in this way. But the upper piece .-, , 



cannot be moved back again, unless the two pieces smooth muscle, and perhaps 

 are intentionally separated from one another by the even in skeletal muscle, as we 

 depth of a tooth. shall see later. 



THE "CATCH" MECHANISM IN SMOOTH MUSCLE 



I use the word "catch" as a translation of von Uexkull's name "Sperrung,"' 

 but it is a matter of difficulty to find one which suggests the complete meaning 

 of the German word. Before trying to explain the idea, we will examine a few 

 experimental facts. 



The strength with which a bivalve mollusc holds its shells together is known to 

 every one who has tried to open an oyster by merely pulling the shells apart. 

 On the face of it, there is nothing to suggest that this fact may not be dm- to the 

 reflex contraction of a powerful muscle. It is found, however, that weights 

 may be arranged to pull continuously, and yet the shells remain firmly closed 

 against a considerable force for many days. To take an example, it requires a 

 tension to be exerted by each square centimetre of the adductor of Dioxinia 

 exoleta equivalent to the weight of 2,400 g. in order to close the shells against the 

 elastic cushion which forces them open. Yet the animal can do this for twenty to 

 thirty days continuously without evidence of fatigue (Pamas, 1910). Considera- 

 tion of such facts led Griitzner (1904) to suggest that the muscle fibres cannot be 

 exerting tensile stress by a continuous excitatory process, but that the fibres must be 

 " hooked up " in some way, by a kind of arrangement similar to a ratchet, and kept 

 in the position to which the shortening process brought them. If we raise a 

 weight to a certain height and hold it suspended, we have seen that cotisadentble 

 work has to be done all the time and that fatigue soon results. But if a bolt 

 is shot out under the weight, so as to support it, it remains in the raised position 

 without any further expenditure of energy on our part. 



The next experiment is one on Pecten which I will give in the words of von 

 Uexkiill (1912, p. 311). "If one takes a normal Pecten out of the water, it gives 

 two or three flaps with its shells before permanently closing them. While it is 

 open, a piece of wood is pushed between the shells, which then close and hit upon 



