NATURE OF SENSE ORGANS 21 



Thus of the two functions that have been attributed to 

 receptors, the capacity to excite action and the ability to 

 initiate impulses for sensation, the former is much the 

 more widely distributed of the two and is without question 

 the more primitive. 



Since sponges are known to possess muscles but are 

 devoid of nervous tissue, it is probable that they represent 

 a type of organization which in point of time preceded 

 that in which the nervous elements arose. So far as can 

 be judged these elements originated in connection with the 

 previously differentiated muscle and as a special means 

 of exciting it to contraction. This earliest nervous mate- 

 rial must have been, therefore, essentially receptive in 

 character and must have served as the source of the more 

 obvious receptors of specialized types. Thus receptors 

 must be regarded as the original form of nervous struc- 

 ture, concerned in the beginning with the simple excita- 

 tion of muscle (activators) and subsequently involved, 

 after the development of the central organs, with that 

 supply of impulses which yields the elements of the intel- 

 lectual life (sense organs). 



The extent to which a natural group of receptors may 

 undergo differentiation and yet maintain a striking degree 

 of mutual interdependence can nowhere be better illus- 

 trated than with the chemical receptors, the organs of 

 smell and of taste. It is from this standpoint that the 

 structure and function of these receptors will be con- 

 sidered in the following chapters. 



4. BlBLIOGBAPHY. 



BEEB, T., A. BETHE, und J. VON UEXKUIX. 1899. Vorschlage zu einer 

 objektivierenden Nomenklatur in der Physiologie des Nervensystems. 

 Biol. CentralU., Bd. 19, pp. 517-521. 



