432 PRINCIPLES OF GENERAL PHYSIOLOGY 



the chain, whereas in homogentisic acid it is in the meta-position. If, however, 

 the OH be first removed from tyrosine, phenyl-alanine is formed, and from this, by 

 de-amination and oxidation, homogentisic acid might be produced thus : 



OH / OH , 



OH J OH 



CHNH 2 



COOH COOH COOH 



I 

 GIL, 



COOH 



Phenyl-alanine. Phenyl-a-oxypropionic Uroleucic Homogentisic 



acia. acid. acid. 



After the action of gravity, then, there is more homogentisic acid found in the 

 root than normally. This is interpreted as follows : homogentisic acid is oxidised 

 to carbon dioxide and water in the resting cells ; under the action of gravity. 

 an " anti-oxidase " is formed, which inhibits the normal oxidation process. Since, 

 as we have seen, the existence of specific anti-enzymes is extremely doubtful, 

 it would be more correct to say that the action of gravity caused the appearance 

 of some substance which retards the action of the oxidase. A similar effect was 

 found to be produced in heliotropic stimulation. It is a result of excitation, 

 not of movement, since mechanical prevention of the latter does not alter the 

 reduction reaction. 



Homogentisic acid plays a part in the normal oxidation of tyrosine in the 

 animal organism (see Garrod, 1909, pp. 41-81), since, in certain inborn errors 

 of metabolism, the enzyme responsible for the further oxidation of homogentisic 

 acid is absent, and administration of tyrosine increases the output of homogentisic 

 acid. In Garrod 's book (p. 78) another mode of conversion of tyrosine into 

 homogentisic acid, through para-oxyphenyl-pyruvic acid, is given. 



SUMMARY 



Automatic cell processes require the provision of means of regulation in two 

 directions, increase and decrease. The former is called "excitation" and the 

 latter, " inhibition." A process which is set into action by influence from without 

 may also be stopped by inhibition of the external influence. 



Processes of a non-living nature are also capable of modification in two 

 directions by external action, as in the familiar case of reversible reactions. 



Strictly speaking, all living protoplasm is able to respond to external changes 

 (" stimuli "), but the name of " excitable " tissues is given for convenience to 

 those which, like muscle and nerve, respond by rapid changes. 



Nerves are especially present for the purpose of conducting a stimulus from 

 the place of application to more distant parts of the organism and bringing 

 the various constituent parts into relation with one another. They really 

 constitute the excitable tissue, par excellence, since they have no other function 

 to perform. 



A nerve when disturbed at a point conveys some sort of change, the 

 propagated disturbance or nerve impulse, along its course to the place where the 

 nerve fibres terminate, and the tissue in which the fibre ends is excited or 

 inhibited according to the particular manner in which the fibre is connected. 

 Of course, the kind of activity which is set going or stopped depends on the 

 cell, nerve cell, muscle fibre or gland cell, etc. 



There is no visible change in a nerve as the impulse traverses it. Indeed, one 

 kind of change only has been definitely shown to take place, an electrical one, in 

 which a spot in activity is electro-negative to one at rest. Certain conclusions as 

 to the nature of the process can be drawn from this fact, taken in connection with 



