502 PRINCIPLES OF GENERAL PHYSIOLOGY 



of two parts, which are distinct members in most Crustacea and united by a 

 movable joint. In this animal, however, in place of a joint, there is a double 

 membrane, whose two components are not very firmly united. In the middle of 

 the membrane there is an aperture, through which the nerve and blood vessels 

 pass. Certain muscles are so arranged that, by a powerful contraction, they 

 separate apart the two layers of the membrane. Thus no soft parts are torn, 

 except the nerve and blood vessels; there is practically no bleeding and the 

 peripheral part of the appendage is rapidly regenerated. 



METHODS 



There are two mammalian preparations which are very useful for the study 

 of reflex action. The " decerebrate," described by Sherrington (1898), for which a 

 cat is best, retains all parts of the central nervous system below the posterior 

 colliculi and shows tonic rigidity of extensor muscles. It is therefore valuable for 

 the investigation of inhibition. The other preparation is decapitated and there- 

 fore spinal only (Sherrington, 1909, 1). It is important that the operative 

 procedures in both cases, especially the section of the crura or the spinal cord, 

 should be done under deep anaesthesia ; a considerable amount of shock is thus 

 avoided. The vagus nerves should also be divided previously ; unless, in the 

 decerebrate preparation, they are required for the purpose of reflexes. 



CONDITIONED REFLEXES 



Pavlov (1910) states that he was struck by the fact that when the physiologist 

 leaves the study of the simpler parts of the central nervous system, which he has 

 investigated by the observation of reflexes, and proceeds to the higher parts, 

 especially to the cerebral cortex, his methods suddenly change. He gives up 

 observation of the relation between external phenomena and the reaction of the 

 organism to them and introduces psychological ideas, derived from his own internal 

 consciousness. 



To extend to the higher centres the method of observing what changes in the 

 organism are correlated with external changes might appear too difficult, but 

 Pavlov has succeeded in doing so to a remarkable degree. The method used is 

 that which he calls " conditioned " reflexes. Unfortunately, up to the present, the 

 experimental results are not easy of access, most of the papers being published in 

 Russian, and it is difficult to follow the train of argument apart from the actual 

 facts. 



The complexity turned out to be less than was expected, so far as the response 

 itself was concerned. The production is easy under the right conditions. The 

 complexity depends on the inhibition produced by events taking place inde- 

 pendently, all of which exert their influence on the newly-formed reflex and must 

 be duly controlled. 



There are two fundamental mechanisms concerned. Firstly, that of temporary 

 association, by which external phenomena are brought into connection with 

 reactions of the organism. And secondly, that of analysers. 



The first, as we have seen in the preceding chapter, is a general property of 

 nerve centres, but becomes more and more complex and modifiable in the course of 

 the evolution of the higher centres. Pavlov makes use of it in a definite manner, 

 which I will endeavour to make plain. 



In the lower centres, the reflexes are of remarkable regularity, as will have 

 been seen in the previous part of the present chapter. They can, as a rule, be 

 reckoned upon to follow a particular stimulus without fail. They are, in fact, 

 "unconditioned." In the higher centres, the result that follows a particular 

 stimulus depends upon a much greater number of conditions, sometimes no obvious 

 result happens at all. Various " temporary combinations " are formed and we have 

 " conditioned " reflexes. It is unnecessary to remark that the difference is really 

 only one of degree, since no reflex can be said to be absolutely unconditioned. 



Now, one of the most essential relationships between the organism and the 

 outer world is that of food. In course of evolution, the means by which the 



