RECEPTOR ORGANS 529 



It is not to be supposed that the only information obtained as to the 

 position of the head in space is derived from the labyrinth. The eyes, as well as 

 the proprio-ceptors of muscles, play a large part in the process, these various 

 receptor organs mutually correcting each other. The reader will probably have 

 noticed how the eye is liable to be deceived by passive movements of the body, 

 when these are unnoticed. A railway train rounding a curve deviates from the 

 vertical owing to the " banking " of the track and, if the line is well laid, it is hard 

 for a passenger to convince himself that the buildings which he sees through the 

 windows are not leaning, although he may know that it is the train itself which is 

 out of the perpendicular. The reader should, perhaps, be reminded that this 

 observation is not possible on such lines as those where it is the custom of the 

 guard to inform passengers in the restaurant car when the train is coming to a 

 curve, so that there may be no " slip between the cup and the lip." 



The astonishing sense of direction possessed by some animals, such as the 

 carrier pigeon, is difficult to explain except as a wonderful memory for labyrinthine 

 sensations received as it is taken from place to place in a basket. 



It appears that gradual change of position is appreciated rather by the 

 statocyst organs and does not readily excite the receptors of the semicirpular 

 canals, which respond to rapid changes to which the relatively inert particles of 

 the former organs would not react sufficiently quickly. 



COMBINATIONS OF SENSATIONS 



It will have been noticed how sensations from different kinds of receptors are 

 combined together to give more accurate and detailed information of external 

 objects. This is particularly the case with the proprio-ceptors of those muscles 

 which move receptor organs in a definite way, such as those of the eye and the 

 hand, when combined with the sensation derived from these sense organs 

 themselves. In this way the notions of space and so on are formed. But here we 

 pass over to the province of psychology, and it is difficult to avoid the use of such 

 words as "sensation," which imply consciousness, in the description of receptors. 

 The reader must understand that nothing further is to be assumed here than the 

 existence of certain nerve impulses passing to particular regions of the brain 

 becoming connected up with other neurones, according to states present in other 

 parts of the nervous system, and finally giving rise to the activation of some 

 effector. 



The discussion of binocular vision and similar aspects of the photo-receptor 

 mechanism is beyond the space permissible here. 



RECEPTORS IN PLANTS 



Plants, like animals, are in relation with changes in their environment, and 

 have also developed means of intensifying and determining the direction of the 

 action of external forces. 



The former mechanism is especially well marked in the so-called "excitable 

 organs," in the narrow sense, where rapid movement exists. The bristles of the 

 leaf of Dionaea are quite entitled to be called receptors ; they make the leaf very 

 sensitive to the contact of insects. A similar phenomenon is to be seen in the leaf 

 stalk of Mimosa pudica, also in the stamens of various species of Centaurea (the 

 blue corn-flower) and in other situations. 



In their sensibility to gravity, whose direction they are able to appreciate 

 plants have an actual separation in space of the receptor and effector as there is 

 in animals. It is the point of the growing rootlet that is sensitive, while tlie 

 response occurs in a region at some distance from this. It may be mentioned 

 here that the proof that the roots of plants are sensitive to gravity was first 

 afforded by Knight (1806), who used centrifugal force to replace gravity and thus 

 obtained a more powerful stimulus. In such cases, it would be quite justifiable to 

 speak of a "reflex action." 



As to the mechanism of the gravity receptors, a similar view was arrived at 



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