30 GENERAL PHYSIOLOGY 



by the digestive juices, are taken up through the cells of the 

 intestinal wall into the body. We know, moreover, that a great 

 part of the ingested fat, after being divided into microscopic 

 globules, is taken into the protoplasmic bodies of the intestinal 

 epithelium-cells by their own activity, while the same cells do not 

 take up other particles of equal microscopic size, such as granules 

 of pigment. But Physiology has not yet learned how this 

 selective faculty of the intestinal epithelium-cells is to be 

 explained mechanically. 



We have seen how in the development of the human body the 

 succession of definite morphological stages up to the complete 

 man, which previously was so mysterious, may be understood 

 naturally from the fundamental law of biogenesis. But it is still 

 a much-debated question how in this development of the cells that 

 arise from the segmentation of the egg some become gland-cells, 

 others nerve-cells, and others epidermis-cells. 



We have learned that the movements of the skeletal bones, the 

 arms, the legs, and the joints, follow purely mechanical and 

 mathematical laws, especially the laws of the action of levers. 

 But the action of the skeletal muscles which causes the move- 

 ment of the skeletal bones is the same puzzle that is mentioned 

 above, namely, the contraction of the muscle-cells. 



From the law of the conservation of energy we know that the 

 heat and the electricity produced by the living body are derived 

 from chemical changes which the ingested food undergoes in the 

 body-tissues. But we do not know at all with what chemical pro- 

 cesses the cells of the various tissues are concerned in the production 

 of this heat and electricity. 



We know, finally, that the higher sense-organs of man are 

 constructed in accordance with the principles of physical apparatus ; 

 the eye, e.g., according to the principle of a camera obscura, so that 

 a reduced inverted image of an object in the external world is 

 formed upon its background according to the laws of the refraction 

 of light. But it is a constant puzzle as to what occurs in this 

 process in the retinal cells and how from them by the mediation of 

 the optic nerves the ganglion-cells in the brain are excited to pro- 

 duce in us the idea of the image. 



This enumeration might be long continued, but what has 

 been said suffices for the recognition of a general fact. Every- 

 where, to whatever branches of physiology we may turn, wherever 

 the gross activities of the body are traced to the activity of the 

 individual cells, we always come upon an unsolved problem. The 

 pessimist, indeed, might be led to maintain with Bunge ('94): 

 " All processes in the organism which may be explained mechanically 

 are no more phenomena of life than are the movements of the leaves 

 and branches of a tree that is shaken by the storm, or the movement 

 of the pollen that the wind wafts from the male poplar to the 



