238 LIFE : OUTLINES OF GENERAL BIOLOGY 



formed is not oxidised— a more economical arrangement is found: 

 alx)ut one-fifth of the lactic acid is converted to carbon dioxide 

 and water, and the energ>' furnished by these reactions is sufficient 

 to reconvert the rest of the lactic acid into hexose-phosphate, its 

 precursor, ready to be used again. The connection between the 

 chemical energy of the oxidation of lactic acid and the physical 

 work done by the contracting muscle-fibre is at first sight very 

 indirect, nevertheless the latter depends entirely on the former. 



In regard to all the bodily functions, it is important to think of 

 them not only along the line of physiological analysis, but as they 

 are expressed in everyday life. Their efficiency cannot be rightly 

 appreciated unless the "Natural History" or ecological aspect is 

 included. We would therefore refer to a concrete instance of the 

 contractile or muscular function. 



One of the most admirable of gymnastic feats among animals is 

 the leap of the salmon. Against a rapid rush of water the fish hurls 

 its heavy body into the air and surmounts a cascade many feet in 

 height. To do this, it has to attain to a high momentum before it 

 takes the actual leap, and we must notice how much of the posterior 

 body of the fish is sheer muscle. The ratio of muscle to the total 

 weight of the body is unusually high. One of the most remarkable 

 leaps on record must be that shown in a photograph published by 

 Dr. David Starr Jordan in Nature for January i6, 1926, representing 

 a Pacific King Salmon (Chinnock or Quinnat) of about 25 lb. in 

 weight, springing over the Punch Bowl Falls in the Cascade Range, 

 Oregon, the height being about 30 feet! 



NERVOUS AND SENSORY FUNCTIONS 



As in other cases, we must restrict ourselves here to a few illustra- 

 tions of the functions of the nervous system and the sense organs. 



THE NERVE IMPULSE.— Few problems in physiology have been 

 more debated than that of the nature of the nerve impulse. The 

 sole of the foot is tickled, and a nerve impulse speeds up to the 

 higher centres in the brain and spinal cord; a "reflex arc" is estab- 

 lished and motor impulses come, by other nerve fibres, to the 

 muscles of the leg; the knee is bent, withdrawing the foot from the 

 irritation. The problem is, what goes on in the nerve-fibre when an 

 impulse travels along it ? English physiologists, notably Keith Lucas 

 and Adrian, have done much to elucidate this question. They have 

 shown, for instance, that the impulses in a single fibre are of 

 constant strength; that is, if the fibre carries a message at all, it 

 carries it at full power ("All or None" Law). Moreover, if an area 

 on a fibre is weakened by anaesthetics, the impulse may, indeed, be 

 choked of! altogether; but, if it manages to struggle through the 



