32 THE HUMAN BODY 



As regards the nutritive tissues it requires especially to be borne 

 in mind that although such a classification as is here given is use- 

 ful, as helping to show the method pursued in the domestic econ- 

 omy of the Body, it is only imperfect and largely artificial. Every 

 cell of the Body is in itself assimilative, respiratory, and excre- 

 tory, and the tissues in this class are only those concerned in the 

 first and last interchanges of material between it and the external 

 world. They provide or get rid of substances for the whole Body, 

 leaving the feeding and respiration and excretion of its individual 

 tissues to be ultimately looked after by themselves, just as even 

 the mandarin described by Robinson Crusoe who found his dignity 

 promoted by having servants to put the food into his mouth, had 

 finally to swallow and digest it for himself. Many secretory cells, 

 too, have no concern with the digestion of food, as for example 

 those which form the various hormones (p. 28). 



4. STORAGE TISSUES. The Body does not live from hand to 

 mouth: it has always in health a supply of food-materials ac- 

 cumulated in it beyond its immediate needs. This lies in part in 

 the individual cells themselves, but apart from this reserve there 

 are certain cells, which store up considerable quantities of material 

 and constitute what we will call the storage tissues. These are 

 especially represented by the liver-cells and fat-cells, which con- 

 tain in health a reserve fund for the rest of the Body. 



5. EXCITABLE, OK IRRITABLE TISSUES. These include those 

 tissues which are especially susceptible to changes in their sur- 

 roundings and are therefore useful in giving to the Body information 

 of what is going on around it. Any change in the environment 

 which serves to arouse response in an excitable tissue constitutes 

 a stimulus. 



6. CONDUCTIVE TISSUES. While most, if not all, of the cells 

 of the Body retain the property of conductivity in some degree, 

 the nervous tissues exhibit it in very high degree. They serve 

 therefore to bring into communication the various parts of the 

 Body. As an incident in the conveying of messages from one 

 part of the Body to another to fulfil the requirements of nervous 

 co-ordination certain nervous structures have the power of modify- 

 ing the messages which pass through them. 



7. MOTOR TISSUES. These have the contractility of the orig- 

 inal protoplasmic masses highly developed. The most important 



