POL YSACCHA RIDES 81 



These relationships are very important, and we shall have occasion 

 to refer to them again in later chapters. 



Melibiose is a galactoside of glucose. It is derived from the trisac- 

 charide raffinose by hydrolysis. 



POLYSACCHARIDES. 



We must now take up the consideration of the Polysaccharides, or 

 carbohydrates formed by the union of more than two molecules of the 

 simple sugars, with the elimination of a corresponding number of 

 molecules of water. 



A few tri- and tetra-saccharides are tolerably well known and 

 defined; of these the most important is Raffinose, CigH^Oie, a trisac- 

 charide which is found abundantly in many plant-tissues and products, 

 particularly molasses, eucalyptus-manna, wheat, barley, fungi, bacteria 

 and yeast. It may be distinguished from cane-sugar by its greater 

 solubility in methyl alcohol, and by the fact that it is split by emulsin, 

 yielding d-fructose and melibiose, while cane-sugar is not attacked by 

 this ferment. Hydrolysis by acids yields first d-fructose and melibiose, 

 then the melibiose is hydrolyzed more slowly, yielding d-galactose and 

 d-glucose. Raffinose does not reduce Fehling's solution. 



Raffinose is not split by animal tissue-extracts nor by any of the 

 digestive juices with the exception of gastric juice which slowly inverts 

 it owing simply to the fact of its acidity and not to any ferment con- 

 tained in the juice. As the gastric contents are only distinctly acid for 

 a brief period during digestion we may infer that this mode of splitting 

 raffinose is of no nutritive significance since it must be of very trivial 

 extent. A portion of the raffinose contained in the food is probably 

 absorbed unaltered and excreted as such in the urine, the remainder 

 with the exception of the very small proportion inverted in the stomach, 

 remains unaltered until it reaches the large intestine (cecum) where it 

 is rapidly inverted by the bacteria which inhabit this portion of the 

 alimentary canal and is thus rendered available for nutritive purposes. 



We here meet with a phenomenon which is yearly growing of greater 

 significance in our eyes, namely the Symbiotic Relationship between 

 the mammals and the bacterial parasites which inhabit their intestines. 

 While the bacterial flora of the intestines constitute a parasitic growth, 

 yet their tenure of the intestine is not wholly to the disadvantage of the 

 host, and through the multifarious enzymes which they produce these 

 organisms render available to mammals foodstuffs which would other- 

 wise be indigestible and excreted unaltered. It is probably for this 

 reason that chickens and rats fed upon a strictly aseptic diet do not 

 grow normally. While in the instance chosen, that of raffinose, the 

 products thus rendered available may not be of indispensible impor- 

 tance to the animal economy, yet in many cases, as for example in the 

 splitting of chlorophyll by the intestinal bacteria, the products which 

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