278 



COMPARATIVE PHYSIOLOGY. 



tritive material, but are useful in fumisLing salts and special 



j substances, as certain compounds of sulphur which, in some ill- 



I understood way, act beneficially on the metabolism of the body. 



,' They also seem to stimulate the flow of healthy digestive fluids. 



i Condiments act chiefly, perhaps, in the latter way. Tea, coffee, 



etc., contain alkaloids, which it is likely have a conservative 



effect on tissue waste, but we really know very little as to how 



it is that they prove so beneficial. Though they are recognized 



to have a powerful effect on the nervous system as stimulants, 



nevertheless it would be erroneous to suppose that their action 



was confined to this alone. 



The accompanying diagrams will serve to represent to the 

 eye the relative proportions of the food-essentials in various 

 articles of diet. 



Fia. 225. — ^Alimentary canal of' embryo while the rudimentary mid-gut is still in con- 

 tinuity with yelk-sac (KOUiker, after BischofE). A. View from before, a, pharyn- 

 geal plates; b, pharynx: c, c, diverticula forming the lungs; d, stomach; ./; diver- 

 ticula of liver; g, membrane torn from yelk-sac; h, hind gut. B. Longitudinal 

 section, a, diverticulum of a lung; &, stomach; c, liver; ce, yelk-sac. 



It is plain that if, in the digestive tract, foods are changed 

 in solubility and actual chemical constitution, this must have 

 been brought about by chemical agencies. That food is broken 

 up at the very commencement of the alimentary tract is a 

 matter of common observation; and that there should be a 

 gradual movement of the food from one part of the canal to 

 another, where a different fluid is secreted, would be expected. 

 As a matter of fact, mechanical and chemical forces play a 



