538 PRINCIPLES OF GENERAL PHYSIOLOGY 



vertebrates, according to the researches of Mosso and Pellacani (1882). It was 

 found that the internal pressure which this organ can withstand without further 

 distension is independent of the actual distension existing at a given time. That 

 is, the length of the muscle fibres of its wall may be very different and yet have 

 the same tension ; or rather, as we should say, in view of the behaviour of the 

 invertebrate muscle, the fibres may be "hooked up" by a catch mechanism at 

 various degrees of shortening. It will be noted that in this case, as in some 

 others to be mentioned later, the motor and catch mechanisms must lie. as far as 

 we know, in the same muscle fibre. 



The next question with which we are faced in the consideration of this 

 prolonged " tonic " contraction of smooth muscle is whether the state is associated 

 with any increase of metabolism beyond the normal one. If the muscle is held 

 in the shortened position by a catch or ratchet mechanism, it would appear that 

 increased metabolism is not to be expected, or of a much less degree than in 

 tetanic contraction. Parnas (1910) has, in fact, made experiments which show 

 that, in the bivalve mollusc, none is to be detected. He loaded mussels (Anodonta), 

 whose adductor muscles had an area in section of O3 sq. cm., with a weight of 

 3,000 g. for three hours and found 110 increase in the respiratory exchange, either 

 during or after the loading. Indeed, if one compares the entire respiratory 

 metabolism of these animals, under the conditions stated, with the increase in that 

 of a skeletal muscle of the mammal, also holding a weight of 3,000 g. per 0'3 sq. cm.. 

 it only amounts to about 0'00003 of the latter, calculated from results on the 

 entire metabolism in man. The anodon muscle uses 0-008 rug. of oxygen per 

 hour, as compared with some 2-8 mg. for the gastrocnemius of the cat (Ver/ar, 

 1912, 1, p. 248). Take another experiment by Pamas on three specimens of 

 Venus, which consumed 3'222 mg. of oxygen in four hours, or 0'805 mg. per hour. 

 Loaded with 1,000 g. each for three hours, the consumption was 0'786 mg. per 

 hour, and, subsequently, without load for three hours, O'Sll mg. per hour. A 

 Pecten consumed, at rest, 0'672 mg. of oxygen per hour; under a load of 500 g., 

 0-679 mg. per hour. 



Bethe (1911) investigated the question in another way and confirmed the 

 view of Parnas. He found that no evidence was to be obtained of fatigue nor of 

 loss of weight in fasting molluscs holding up a weight for a considerable time. 

 If the consumption of carbohydrate had been comparable with that of cros 

 striated, skeletal, vertebrate muscle, an amount greater than the weight of the 

 entire animal must have been burnt up. 



An interesting calculation is made by Bethe on the tonus of the arterioles in 

 a mammal. If the mechanism were like that of the skeletal muscle, one-sixth to 

 one-quarter of the whole resting metabolism of the animal would be in the 

 arterioles. 



The hardness of a muscle is proposed by Noyons and von Uexkiill (1911), 

 as a test of the tonic activity of the catch mechanism. In the leech, the same 

 length of animal may in one case be under a tension of 10 g., in another of 70 g., 

 and, to the eye, they have much the same appearance. Tested by the apparatus 

 of the above investigators, the greater hardness of the latter is made apparent. 



Although it seems established that certain muscles are able to hold up, by means of 

 the "catch" mechanism, a weight for a considerable time without appreciable consumption 

 of energy, the work of Cohnheim (1912, 3) on Xi/titiicu/tt*, and of Cohnheim and von Uexkiill 

 on the leech (1912), showed that, when loaded, the energy consumption is greater in these 

 animals than when unloaded. But it is plain that it is difficult to be certain that there 

 is no reflex effect on other muscles or organs, on account of the abnormal conditions present. 



An appropriate form of apparatus for the investigation of the phenomena of tonus, 

 especially in the snail, is described by Jordan (1908, 1912). 



Of course, the comparison of the mechanism in question to that of a catch 

 or ratchet is only intended to assist the reader in grasping the mechanical 

 conditions present, which are similar in both cases. As to the actual process 

 itself, hypothetical suggestions only can be made in the present state of knowledge. 

 The state of tension into which a skeletal muscle of the vertebrate is put by 

 stimulation, passes off automatically when the stimulus is removed. Whatever 

 may be the cause of this increased tension, whether the setting free of some 



