100 OF FOOD, AND THE DIGESTIVE PROCESS. 



blood with Bischoff (highly) at 7.7 per cent, of the whole weight of the body, 

 the total quantity of iron contained in the blood of a man, weighing,about 

 150 Ibs., would be 45 grains. The normal proportions of all these sub- 

 stances are essentially maintained by means of the excretory apparatus, 

 which filters off (so to speak) any surplus, it being through the urinary 

 organs that they are chiefly eliminated. And it is by them too that the 

 normal proportion of water in the blood is maintained; the Malpighian 

 apparatus of the kidneys apparently acting as a kind of safety-valve, 

 through which any surplus that remains after the cutaneous, pulmonary, 

 and intestinal exhalants have performed their appropriate duties, is allowed 

 to make its escape. 



CHAPTER V. 



OF FOOD, AND THE DIGESTIVE PROCESS. 



1. Of Food, its Nature and Destination. 



58. FROM the foregoing observations regarding the composition of the 

 tissues and fluids of the body, it may be concluded that the substances which 

 are required by animals for the development and maintenance of their fabric 

 are of two kinds, the Organic and the Inorganic. The Organic alone are 

 commonly reckoned as aliments: but the latter are really not less requisite 

 for the sustenance of the body, which speedily disintegrates if the attempt 

 be made to support it upon any Organic compounds in a state of purity. In 

 all ordinary articles of diet, however, the Inorganic matters are present in 

 the requisite proportion ; aud hence it is that the necessity which exists for 

 their employment has very commonly escaped notice. 



59. The Organic compounds usually employed as food by Man are partly 

 derived from the Animal, and partly from the Vegetable Kingdom ; and they 

 may be conveniently arranged under the three heads already mentioned of 

 the Saccharine, the Oleaginous, and the Albuminous under which last 

 gelatin is included. There are many other substances, however, which, 

 though truly alimentary and consumed to a considerable amount, cannot be 

 legitimately placed under either of the above heads such are, for example, 

 the vegetable acids, aud pectin, or vegetable jelly. The compounds belong- 

 ing to the Saccharine group 1 consumed as food, though existing in small pro- 



1 By far the most important of the hydrocarbonaceous compounds is starch, which 

 exists in the roots, tubers, stems, leaves, fruit, and seeds of plants, both as an amor- 

 phous and as a morphological constituent. In the former condition it is compara- 

 tively rare, but in the latter it appears in the form of separate granules, presenting, 

 as may be well seen in potato starch, a series of concentric laminae arranged around 

 an eccentrically situated point. Every starch-granule! consists of three kinds of 

 substance of f/ranuloae, which is colored blue by iodine; of erythrogranulose, which 

 is colored red by it; and of cellulose, which is not colored by it. If iodine be added 

 to a starch-granule, it becomes of a deep-blue tint, because the granulose is by far 

 the! hirgest constituent; but if very little iodine be added the granule! becomes of a 

 reddish color, because the erythrogranulose has the strongest atlinity for it, and 

 sei/.es upon it. On boiling with a "2 per cent, solution of sulphuric acid, starch is 

 rendered soluble, and is converted into Nasse's nmi</ulii>, which still turns blue with 

 iodine. On longer boiling it undergoes a further change into erythrotlr.rtrin, or ordi- 

 nary dextrin, wliieh colors red with iodine; and after a still longer time it becomes 

 converted into another kind of dextrin, achroodextrin, which tastes sweet and does 

 'not color with iodine, and into a kind of sugar termed /////cu.sr (starch sugar, dextrin 

 sugar, grape sugar). All the abovementioned bodies rotate the plane of polarization 



