PHYSIOLOGY 



161 



protoplasm a machine for the transformation of 

 potential energy into energy of motion. This 

 machinery is constantly breaking down and being 

 repaired, the protoplasmic matter is continually 

 being replaced by new matter similarly combined. 

 But, as the protoplasm is extremely complex, 

 the simpler substances of the food have to be 

 combined and recombined in a series of stuffs 

 of increasing complexity until the complex living 

 matter itself is formed. These combinations are 

 supposed to be due to a ferment-like power of 

 the protoplasm. This power it is which makes 

 growth possible i.e. the actual increase in 

 amount of protoplasm. The growth of a crystal 

 out of its solution is probably a process not utterly 

 unlike, though much simpler. Growth of a crystal 

 may seemingly be endless, but growth of a cell 

 never proceeds beyond a certain point, when the 

 process known as Cell-division occurs. The mass 

 of protoplasm divides into two halves, and each 

 half goes on to live as before. The necessity for 

 cell-division arises partly from the conditions of the 

 food-supply. Food is absorlied through, the surface 

 of the cell, but with growth the mass to be fed 

 increases faster than surface ; therefore starvation 

 must occur at a certain stage of growth unless the 

 cell divides. The higher animals are built up of 

 numberless cells which have all arisen, by division, 

 from a single cell, the ovum ; but instead of 

 becoming separated they have all kept together, 

 joined probably by strands of protoplasm. The 

 cells are massed into tissues and the tissues into 

 organs, the organs having special functions. This 

 difference in the behaviour of the cells of different 

 parts of the body is known as Division of Labour 

 (q.v. ). We can form some idea of its origin. Imagine 

 a cell to divide many times, but the daughter-cells 

 to remain loosely joined together ; the outer and 

 inner cells would live under different conditions 

 and would assume different functions. The whole 

 story of the evolution of life, lioth in the origin of 

 individual forms and in the growth of nations, is 

 simply the process of the division nnd organisation 

 of labour. For just as an organism is a collection 

 of cells, each having its own life, yet all liound 

 together for mutual service, so is a nation a 

 collection of individual men and women. And as 

 tin 1 perfection of an animal is measured by the 

 completeness of the division of labour among its 

 cells, so is the civilisation of a nation measured 1>\ 

 the harmony of organisation of its labour. Further, 

 just a* there have Ix-en many .species of animals 

 which have appeared, lived for a time, and then 

 given place to higher species, so there have been 

 civilisations which have flourished for a time and 

 then died away. Any fairly complex civilisation 

 will serve as a type of the division of labour 

 in the body of one of the higher animals. First 

 there are the persons concerned in the getting 

 of food, like the limbs and mouth of an animal. 

 Then the food is prepared for use by other labourers; 

 such are the digestive organs of the animal. 

 The food has to be distributed to all memliers of 

 the community by merchants and carriers ; the 

 blood and the blood vessels perform this function. 

 The whole community lias to IK.- warned of dangers, 

 directed ami governed, anil made to act harmoni- 

 ously by the statesmen of a nation ; the same 

 things an- done by the sense-organs, brain, and 

 nervous system of an animal. 



We have already noted that the source of all the 

 energy of an animal lies in its food. We know- 

 that this is either burned as it were within the 

 tissues, used as fuel for the, protoplasmic machinery, 

 or used to keep that machinery m repair; in either 

 ' the food stuffs have to lie prepared before they 

 can be. used. Such preparation is called rUgenfiim. 

 which consists in making the solid food-stuffs 

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soluble. The digested food is absorbed into the 

 blood, and all of it, except the fat, is carried direct 

 to the liver. This organ, amongst other functions, 

 regulates the composition of the blood ; thus, it 

 stores the sugar in its cells, and gives it out as the 

 other tissues require. Muscular tissue is the great 

 consumer of sugar, which is to the cells what 

 coal is to the steam-engine. But there is another 

 and most important food-stuff that requires no 

 digestion. This is oxygen, which is needed by the 

 protoplasm for its life, and also for the burning of 

 fuel within the living machinery to get heat and 

 energy of motion. The oxygen is held in the 

 Blood (q.v.) by means of a special substance which 

 greedily absorlis it from the air in the lungs, 

 and yet gives it up readily to the protoplasm 

 of the tissues. The blood as is well known 

 circulates round and round the Ixxly, pumped by 

 the heart. It is a stream of food material by 

 which each cell of the tissues is fed. For each cell 

 is close to a capillary, which is a very thin walled 

 blood-vessel, through which the fluid food oozes, 

 and thus bathes the tissues. The matter which 

 has thus passed out of the blood- vessels is collected 

 into another system of vessels, the lymphatics, and 

 eventually emptied into one of the great veins. 

 The lymph stream is also the drain into which is 

 thrown by each cell the waste products of its 

 activity. "The carbonic acid that is formed in the 

 tissues is carried away by the blood, and escapes out 

 of the system from the lungs. Some of the useless 

 water is also got rid of in the sanie way, and some 

 more of it is sweated out by the glands in the 

 skin ; the rest is filtered out of the blood by the 

 kidneys. There are many other waste matters 

 liesides carbonic acid and water. These are to a 

 large extent prepared for excretion in the liver, 

 and to some extent actually taken out of the blood 

 by that organ, being poured into the intestine, 

 mixed with other matters, dissolved in a fluid 

 called Bile (q.v.). They are all taken out of the 

 blood by the kidneys, and cast out of the body 

 along with the water filtered out by the same 

 organs, as mine. 



This tini-hcs our sketch of the lalKiurs of the 

 inferior memliers of the cell community. The more 

 skilled workmen are the cells of the sense-organs 

 and the nervous system ; these are described in 

 other articles. As has been noted, their function 

 is to inform the community of what is going on in 

 the outside world, and to Keep in harmony all the 

 diverse labours of the various organs. 



The function of Reproduction is treated in that 

 article. There remains only the duration of life 

 to consider ami the fact of death. The general 

 theory of the length of life is set forth in the article 

 on longevity. The usual view of death is that it is 

 inherent in living matter ; that there is some cause 

 which renders the cells of the body, after a certain 

 period of life, and after a certain number of 

 divisions, less and less able to nourish them- 

 selves, to continue dividing, and to keep the body 

 in repair. Recently it has been suggested by 

 Weismann that death has been evolved by natural 

 selection as a preventive against the continuance 

 in life of maimed individuals (for no one can escape 

 slight injuries) that would be only a burden to the 



For Comparative Physiology, see the artiales on 

 the various functions and groups of animals. 



Tkf, History of Physiology , in its limited sense 

 as the study of the life-processes of individual 

 organisms, is the history of an ever-deepening 

 analysis. The science begins with the study of the 

 general habits of animals ; the life-processes are 

 then resolved into the functions of the various 

 organs, the organs are analysed into their com- 

 ponent tissues, the tissues into cells (see CELL), 



