ESSENTIALS OF PHYSIOLOGY. 



CHAPTER I. 

 INTRODUCTORY. 



EVERY living structure is derived, so far as our present knowledge 

 goes, from another living structure and exhibits certain well-marked 

 features. It takes up non-living material and builds it up in a more 

 or less modified form into its own framework, it has the power of 

 growing up to a certain limit, and it is capable of giving rise to other 

 living organisms like itself. Further, it has the property of irritability, 

 that is, it may be affected by a change in its immediate surroundings 

 which is called a stimulus, and to which it responds by some change 

 in itself, usually movement or secretion. These manifestations of life 

 form the subject-matter of the science of Physiology, which naturally 

 falls into two divisions, Vegetable and Animal. With the former of 

 these, however, we are not here concerned. 



The functions of animal life may be studied in their most primitive 

 condition in a unicellular organism, such as amoeba. This minute 

 creature may be observed under the microscope to enfold particles of 

 food material, to assimilate what is useful in these particles, and to 

 reject what is useless; it may be seen to respond to chemical or 

 mechanical stimuli by movement, sometimes contracting into the 

 smallest bulk by becoming spherical, at other times protruding part 

 of its substance and transferring itself, as it were, into the protruded 

 part, and so changing its position. It may also be observed at a 

 certain stage in its life to divide into two, and each of the young 

 arnrebse so produced grows until it too gives rise in its turn to another 

 generation. 



In the higher animals the body is composed of a multitude of cells, 

 and this complexity of structure is accompanied, as in a community 

 of persons, by a specialisation of function whereby certain cells are 



