BOOK II, 



SPECIAL PHYSIOLOGY. 



CHAPTER IV. 



OF FOOD, AND THE DIGESTIVE PROCESS. 

 1. Sources of the Demand for Aliment. 



406. THE dependence of all Organized beings upon food or aliment, 

 must be evident from the facts stated in the preceding portion of this 

 Treatise. In the first place, the germ requires a large and constant 

 supply of materials, with which it may develope itself into the perfect 

 being, by the properties with which it is endowed. In all but the 

 lowest tribes of Plants, we find the materials required for the earliest 

 stages of the process prepared and set apart by the parent. Thus in 

 the seedy the germ itself forms but a small proportion of the whole 

 substance, the principal mass being composed of starchy matter, which 

 is laid up there for its nutrition ; and the act of germination consists in 

 the appropriation of that nutriment by the germ, and the consequent 

 development of the latter, up to the point at which it becomes inde- 

 pendent of such assistance, and is able for itself to procure, from the 

 soil and atmosphere that surround it, the materials for its continued 

 growth. So in the egg of the Animal, the principal mass is composed 

 of Albumen and oily matter ; the germ itself being, at the time the egg 

 is first deposited, a mere point invisible to the naked eye ; these mate- 

 rials serve as the food or aliment of the germ, which gradually draws 

 them to itself, and converts them into the materials of its own struc- 

 ture ; and at the end of a certain period, the young animal comes forth 

 from the egg, ready to obtain for itself the food which is necessary for 

 its continued increase in size. 



407. In many instances among the lower animals, the form in which 

 the young animal emerges from the egg is very different from that 

 which it is subsequently to assume ; and the latter is only attained by 

 a process of metamorphosis. This change has been longest known, and 

 most fully studied, in the case of Insects and Frogs ; which were for- 

 merly thought to constitute an exception to all general rules in this 

 respect, the Insect coming forth from the egg in the state of a Worm, 



