142 HABIT AND INTELLIGENCE. [cHAP. 



circula- in any similar way depend on nervous agency. Thus, as 

 reciprocal. J^^^ Stated, the nerves of a part are paralysed by tying the 

 artery that supplies it ; but the circulation of a part is not 

 paralysed by cutting the nerve-trunk that sup)plies it. 

 Opposite Eecent experiments on cold-blooded animals have shown 

 of blood^ ^ "^6ry remarkable contrast between the relations of blood 

 and nerve and nerve to muscular fibre. We know that the stimula- 

 tion of a nerve tends to cause the muscle in connexion 

 with it to contract. It is now ascertained that the supply 

 of fresh arterial blood tends to cause the muscles that it 

 bathes to retain the elongated state ; and when the supply 

 of blood is cut off, the muscles tend to contract spon- 

 taneously.^ In other words, blood tends to keep the 

 muscles elongated and relaxed, and nervous action makes 

 them contract. To state these facts in the language of the 

 dynamic theory : — The blood undergoes oxidation in the 

 lungs : in the process of oxidation energy is liberated, 

 part of which is carried by the blood to the muscles, and 

 supplies them with their charge of static vital energy.^ 

 So long as they remain elongated, the muscles retain this 

 charge of static vital energy ; but when a nervous stimulus 

 comes and makes them contract, this vital energy is 

 , transformed into motor energy. 



It is to be observed, that the elongated or relaxed state 

 of a muscle is that in which it contains a charge of energy, 

 and the contracted state is that in which the energy 

 has been parted with. This distinction is one on which 

 emphasis must be laid, for it is very important, and by no 

 means obvious. It is totally unlike that which obtains in 

 an elastic body, such as an india-rubber cord ; for the 

 india-rubber contains a charge of energy while it is strained, 

 but none while it is relaxed. 



It is also proved by the foregoing facts that the blood 



^ See Dr. Norris's Keport on Muscular Irritability, British Association 

 Report, Nottingham, 1866. This law, I believe, was first stated by 

 Dr. Radcliffe. (Carpenter's Human Physiology, p. 680.) 



2 See p. 97. The theory that the motor energy produced by any muscle 

 is due to the oxidation of that muscle at the very moment, is, I think, 

 conclusively disproved by the experiments of Fick and "Wislicenus. (Sec 

 Philosophical Magazine, June 1866.) 



