34 PHYSIOLOGY 



position of this internal medium is maintained constant by the activity of the 

 cells in contact with it, and the stress of sudden changes in the chemical com- 

 position of the surrounding medium is borne entirely by the outer protective 

 layer of epiblast cells. These are rendered more or less impermeable by the 

 secretion on their surfaces of a cuticular layer, and only such of the con- 

 stituents of the surrounding medium are allowed to enter the organism as can 

 be utilised by it for building up its living protoplasm. Out of the ccelom 

 is later on formed a circulatory system which, by the circulation of the ccelo- 

 mic fluid or of blood through the whole body, can procure a still more per- 

 fect uniformity in the chemical conditions to which every cell is exposed. It 

 is not till much later that the organism achieves an independence of external 

 conditions of temperature. In the mammalia, by means of the reactive 

 nervous system, the heat produced in every vital activity by the chemical 

 changes of combustion and disintegration is so balanced against the heat 

 lost through the external surface to the environment that the temperature of 

 the internal fluid is maintained practically constant. One of the main results 

 of the differentiation of function and structure is therefore a gradual setting 

 free of the majority of the cells of the body from the influence of variations in 

 the environment ; and in the highest type of all animals, in man, this inde- 

 pendence of external conditions is carried to a much further extent by 

 conscious adaptations, such as the use of clothes, dwellings, artificial heating, 

 and so on. 



The differentiation of the cells which compose the organs of the body is 

 determined in the first place by the different conditions to which they are 

 exposed in virtue of their positions in the course of development. All the 

 higher animals may be considered as built in the form of a tube, the external 

 surface of which is modified for the purpose of defence and for adaptation to 

 changes in the environment. From this layer there are developed not only 

 the protective cuticle, but also the organs of motor reaction, namely, the 

 special senses and the nervous system. The internal surface of the tube 

 is modified for purposes of alimentation. From it are developed all those 

 structures which serve for the digestion of the food-stuffs, for their absorption 

 into the common circulating fluid, for their elaboration after absorption, and 

 their preparation for utilisation by other cells of the body. Between these 

 two surfaces are situated the supporting tissues of the body as well as the 

 organs for the conversion of the potential energy of the body into motion and 

 work, namely, the muscles. Here also is the ccelum or body cavity, repre- 

 sented in the higher animals by the pleural and peritoneal cavities. The 

 alimentary canal projects for a considerable part of its course into this coelum, 

 being attached to the body wall only by one side. From the ccelom is also 

 developed the blood vascular system, surrounded by contractile and con- 

 nective cells which maintain a constant circulation of the blood throughout 

 the body. By this differentiation the body becomes divided into a number 

 of organs, each of which is composed of like cells, modified for a common 

 function and bound together by connective tissue, the latter serving also to 

 carry the blood-vessels which convey the common medium for the working 



