THE RECEPTORS AND EFFECTORS 79 



ous experience. In higher vertebrates, as a rule, all but the 

 simplest and most elementary exteroceptive activities are indi- 

 vidually acquired, variable, non-hereditary, plastic behavior 

 types. The elements of which these acts are made up are, of 

 course, necessarily present in the inherited reflex pattern; but 

 the pattern according to which these elements are combined is 

 not wholly predetermined in the hereditary organization of the 

 species (pp. 31, 301). 



With these principles in mind, let us now undertake an anal- 

 ysis of the human receptors and of the nervous end-organs re- 

 lated to their effectors, or organs of response. The following list 

 is by no means complete and is in some parts merely provisional. 



I. SOMATIC RECEPTORS 



These are concerned with the adjustment of the body to external or 

 environmental relations. 



A. THE EXTEROCEPTIVE GROUP 



The sense organs of this group are stimulated by objects outside the 

 body and typically call forth reactions of the whole body, such as locomo- 

 tion, or of its parts, so as to change the relation of the body to its environ- 

 ment. This group includes a system of general cutaneous sense organs, 

 some organs of deep sensibility, and some of the higher sense organs. The 

 cutaneous exteroceptors comprise a very complex system whose analysis 

 has proved very difficult. The conclusions presented in the paragraphs 

 which follow are based chiefly upon the observations of von Frey, Henry 

 Head, and Trotter and Davies. The correlation of the data of physiological 

 experiment with the anatomical structure of the cutaneous end-organs is 

 still somewhat problematical and the assignment of end-organs here to the 

 various cutaneous senses should be regarded as provisional rather than as 

 demonstrated. 



1. Organs of Touch and Pressure. These fall into two groups, those 

 for deep sensibility (pressure) and those for cutaneous sensibility (touch). 

 The deep pressure sense is served by nerve-endings throughout the tissues 

 of the body and is preserved intact after the loss of all cutaneous nerves. 

 Most of the functions of the deep sensory nerves belong to the propriocep- 

 tive and interoceptive series (see below), but some exteroceptive functions 

 are here present also. The latter are probably related chiefly to the 

 Pacinian corpuscles and similar encapsulated end-organs. The Pacinian 

 corpuscle has a central nerve-fiber enclosed in a firm lamellated connective- 

 tissue sheath (Fig. 22). By these end-organs relatively coarse pressure 

 may be discriminated and localized (exteroceptive function), and movements 

 of muscles and joints can be recognized (proprioceptive function). The 

 sensory fibers concerned with the deep preesurexsense-are distributed through 

 the muscular branches of the spinal nerves in company with the motor 

 fibers. The point stimulated can be localized with a fair degree of accuracy, 



