WORK THE MUSCLES 99 



Muscular Mechanisms of Various Kinds 



It is not only for the purpose of bringing about effects in the 

 outer world that muscles are made use of. Those concerned with 

 breathing and in speech, and those by which the eyes are moved, 

 may be referred to. The importance of the latter will be seen 

 later. 



That kind of muscle called smooth or involuntary has been 

 mentioned already in connection with the movements of hollow 

 organs, such as the alimentary canal, and its general properties 

 have been described. The heart and blood vessels will be dealt 

 with in a subsequent chapter. 



Posture Phenomena 



There are some rather remarkable phenomena exhibited, especi- 

 ally by involuntary muscle, but also in a certain way by voluntary 

 muscle. They are not yet completely understood, but are of much 

 importance. If we try to keep a weight raised with the arm out- 

 stretched, we soon become aware that a continuous expenditure of 

 energy is required. On the other hand, a bivalve mollusc, such as 

 an oyster, is able to keep its shell firmly closed, even when continu- 

 ally pulled upon by a weight, for a long time without signs of fatigue 

 or evidence of consumption of material. There appears thus to be 

 a possibility for certain muscles to maintain themselves at various 

 lengths, which oppose resistance to stretching, but without the 

 presence of a state of tension. It is as if they had become fixed at 

 a particular length, as by freezing, and that a kind of thawing pro- 

 cess was necessary in order to restore them to their original state. 

 We may picture the state as being analogous to the holding up of 

 a weight, after it has been raised to a height by the expenditure of 

 energy, by slipping a support underneath it. It does not fall again 

 until the support is removed. The process of relaxation in the 

 muscle, corresponding to the removal of the support, is brought 

 about by the stimulation of a nerve, and does not take place other- 

 wise ; this nerve is a different one from that which induced the 

 shortening. Thus, if certain nerves supplying the closing muscle 

 of the mollusc be cut while the muscle is in a state of contraction, 

 it remains permanently at this length, unless the end of the nerve 

 in connection with the muscle is stimulated, and then relaxation 

 occurs. These properties are exhibited by the urinary bladder 

 of the vertebrate in a striking way. If this organ were like an 

 india-rubber ball, the greater the filling the higher would be the 

 tension of the walls and the pressure inside it. But this is not 

 the case. It may possess very various degrees of tension with the 



