CHAPTEE VI 



THE INTESTINE AS AN ORGAN OF EXCRETION 



CONTENTS. 1. Physical characters and chemical composition of faeces and 

 intestinal gases. 2. Alimentary residues and waste products in faeces, while 

 taking food and in fasting. 3. Formation of faecal masses a function almost 

 exclusively confined to small intestine. 4. Theory of normal human faeces. 

 5. Toxicity of faeces. 6. Mechanical and chemical functions of caecum. 

 7. Mechanism of defaecation. 8. Innervation. Bibliography. 



WE have seen (in Chapters III. and IV.) that the character 

 common to all the chemical processes carried on in the alimentary 

 canal is the hydrolytic cleavage of the larger molecules of the 

 food-stuffs, by which these are reduced to smaller groups of atoms, 

 and become more soluble and diffusible. They are thus converted 

 into suitable material for the anabolic chemical activity of the 

 living protoplasm, and the synchronous and homologous (construc- 

 tive or reintegrative) changes within its cells. 



In the last chapter, in discussing what is known of these 

 hidden processes, we followed the three different groups of organic 

 food-stuffs from the alimentary canal to the blood, and from the 

 blood to the liver, as well as to the other tissues and organs. We 

 found that the anabolic processes (both chemical and cytological) 

 precede or succeed or in any case are intimately connected and 

 associated with processes of the opposite or katabolic nature. In 

 these the nutrient substances, on penetrating into the protoplasm, 

 or before entering the structure of the living matter, for the most 

 part undergo a succession of retrograde chemical transformations, 

 by which they are ultimately reduced to waste products, destined 

 to be eliminated from the body as useless or injurious. This 

 disintegrative work of the tissues, in which the potential energy 

 of the restitutive substances of alimentary origin is liberated 

 under various forms, necessarily involves a certain consumption 

 and expenditure of living matter, which increases the sum of the 

 katabolic products. 



The ultimate and simplest waste products of the animal 

 economy are : 



(a) Urea, which is derived entirely from the consumption of 

 proteins, and is formed chiefly, if not exclusively, by the liver. 



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