ioo INTRODUCTION TO GENERAL PHYSIOLOGY 



same degree of filling ; or conversely, various degrees of filling 

 may coincide with the same tension. The muscle of its walls has 

 the power of altering its length to accommodate the contents 

 without changing its tension, just as we can voluntarily adjust 

 the grasp of the hand so as to exercise the same pressure on a 

 large or on a small ball. There is also reason to believe that 

 the muscular coat of the small blood vessels, which prevents their 

 over-distension by the pressure of the blood, has properties of the 

 same kind. 



Something of the same kind is shown by the voluntary muscle 

 of the vertebrate ; but in this case it is more directly brought about 

 through the nervous system. After removal of certain higher parts 

 of the brain, it is found that a limb offers resistance to a change in 

 position, because some of its muscles are in a state of shortening. 

 When this resistance is overcome, the limb remains in the position 

 in which it has been placed, although its own weight may have to 

 be held up against gravity. This reaction is due to the stimulation 

 of certain nerves in the muscle substance, which convey messages 

 to the nerve centres, and the result is a reflex stimulation of nerye 

 fibres causing the peculiar form of contraction. There is evidence 

 that this " postural " state requires the expenditure of much less 

 energy than the voluntary production of the same degree of 

 shortening. It appears that conditions of this kind are to be met 

 with in some forms of " contracture," met with after injury, although 

 not directly due to it (P., pp. 333, etc.). 



The suggestion has been made that it is the sarcoplasm of the 

 muscle that is responsible for the phenomena spoken of in the pre- 

 ceding paragraph. But the proof is not complete. 



Energy for Other Purposes 



From various statements in the previous pages of this book, it 

 will be realised that a supply of energy is needed for such purposes 

 as raising osmotic pressure, chemical reactions in which potential is 

 raised, and so forth. 



A useful index of the amount of energy required by an organ 

 is the oxygen consumed by it, since oxidation is the source of the 

 energy. This can be found by comparing the oxygen present in 

 the blood going to the organ with that in the blood leaving it in 

 a given time. This has been done in the cases of the secreting 

 glands and the voluntary muscles already mentioned. 



All living cells are found to consume oxygen, although it is 

 not always obvious for what purpose they require energy. It has 

 been suggested that it is to prevent diffusion, to maintain the 

 integrity of membranes, and other purposes of this kind. 



