68 



PAET II.-ANATOMY, 



1 



Hf; CHAPTER I. CELLS. 



Protopksm. 62. The bodies of plants and of 



animals consist primarily of protoplasm ; this is a colourless 

 substance, the consistency of which varies with the quantity 

 of water it contains from that of a viscid fluid to that of a 

 firm substance almost like wax. Protoplasm always contains 

 carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus 

 and iron. When heated strongly protoplasm gives off water 

 and then it blackens and gives off ammonia. If heated to 

 red heat an ash is left consisting of small quantities of mineral 

 substances. Protoplasm, however, is a living substance which 

 can be easily killed by too high a temperature, by small 

 quantities of poisons and by other factors. It must not be 

 regarded as a simple chemical compound, and we cannot 

 produce protoplasm artificially by collecting and mixing to- 

 gether all the elements which appear to enter into its compo- 

 sition. 



Some plants exist which consist of only a naked mass of 

 protoplasm. Such a mass of living protoplasm is found to be 

 capable of creeping movements, portions of the protoplasm 

 being pushed out which draw the remainder of the protoplasmic 

 body after them. It is also capable of taking up and absorb- 

 ing bodies containing nourishing materials with which it 

 comes in contact, of building up new protoplasm and thus of 

 growing and increasing in size, of respiration, i.e. of absorbing 

 oxygen from the air and giving off carbon dioxide, and finally 

 of throwing off pieces of itself which are capable of an inde- 

 pendent existence. In addition to these powers of movement, 

 nutrition, growth, respiration and reproduction, living pro- 

 toplasm, in common with all living matter, possesses the 

 remarkable characteristic of irritability, i.e. it is sensitive to, 

 and capable of re-acting in response to, a stimulus. The 

 quality is most easily recognised in cases where the response 

 is greatly out of proportion to the stimulus. If the leaves of 

 the sensitive plant, Mimosa pudica, are lightly touched, the 

 leaflets and pinnae all close together and the whole leaf 

 sinks downwards. In this case contact with another body 

 has supplied the stimulus to which the living protoplasm has 



