DIGESTION IN OTHER ANIMALS.

313

this latter it passes, as in man, through the pylorus into the intestines, which are greatly longer than in man.

Esophagus

Cardia

8d Stomach

Fig. 108.

1111

The reason of all this complication in the digestive apparatus of the ruminating animal, is the difficulty of grinding down, and then of extracting, the whole of the nutritive matter from the kind of vegetable food on which the animal lives. Hence the food is longer detained in the alimentary canal, and is subjected to a more thorough process of subdivision and exhaustion, before it is allowed to escape from the body.

The chemistry of comparative digestion is indeed rich in interest and instruction; and, did my space permit, it were easy to multiply illustrations of the way in which the instruments and means of digestion are adapted in every animal to the circumstances in which it is placed, and to the habits of life in which it is intended to indulge.

VOL. II.-14

In all animals, however, the end or purpose of digestion. is the same, to provide materials for building up its body to a full size, and afterwards for enabling it to discharge its various living functions, without permanent loss of its own weight or substance.

CHAPTER XXXI.

THE BODY WE CHERISH,

The body and its habits an assemblage of chemical wonders.-Change of the food in its passage from the mouth to the lacteals.-Globules or corpuscles of the chyle.— The blood corpuscles; their form and composition.—Mineral matter within and without the corpuscles.-The corpuscle is an independent microcosm.-Selecting power of the parts of the body.-How the whole system is kept in working order.--Activity of the vessels which remove waste matter.-Change of the capacity of the blood for heat in passing through the lungs.-How this affects the warmth of the body.-Other provisions for comfortable warmth.-Craving for special kinds of food.-How this is artificially met.-The nature of the water we drink may modify natural cravings and natural diet.-The potato and water of Ireland.-Instinctive choice of beverages and condiments.-Case of salt; how instinct regulates the use of this substance.-Examples in South-western Africa and in Siberia.-Susceptibility of the body to the action of very minute portions of matter.-The narcotics, the beverages, the odours, and the miasms.--Influence of light upon the body.-The structure, functions, and special composition of the grey and white parts of the brain. The rete mucosum.-The chemistry of all parts of the body deserving of intelligent and reverential study.

NEARLY all the functions and habits, natural and acquired, the chemical history of which has formed the subject of the preceding chapters, have a relation more or less direct with the welfare and comfort of the body. Besides ministering to its necessary wants, we nourish and fondly cherish it. And in attempting to pleasure and pamper, we often injure it. This arises from our possessing, for the most part, too imperfect a knowledge of its vital wants and functions. We

are too little familiar, also, with the substances we daily use or occasionally indulge in, or with which, in external nature, we cannot avoid coming into contact. And with this ignorance of the things themselves, is necessarily associated a similar ignorance of the effects they are likely to produce upon the system.

This want of knowledge is by no means surprising, seeing that the whole grown-up man-the body and its habits together-may be described as an assemblage of chemical wonders. Besides the main features in his chemical history which have been already illustrated, there are a thousand others of a less general kind, the study of which is not only rich in the discovery of wise contrivances, so to speak, but is pregnant also with practical instruction. To some of these minor points I propose to devote the present chapter.

We have already seen how many curious circumstances attend the food in its progress from the mouth to the bloodvessels. The teeth grind it fine, and the tongue mixes it with the saliva. The saliva, on the watch to be useful, rushes out and makes the mouth water whenever savoury food is spoken or even thought of. It flows most copiously, however, while we chew and while we are digesting. In doing so, the saliva not only moistens and seasons the food, but mixes up with it the substance ptyalin, which converts its starch into sugar, and is essential to the healthy progress of digestion Then from the coats of the stomach exudes the gastric juice-also most copiously when there is most work to do. This fluid brings with it the peculiar substance pepsin, which renders soluble the gluten and flesh of the food. When this solution is accomplished, the gastric juice ceases to flow, and the liquid food moves forward to the smaller intestines. Here the sour chyme is mixed with three fluids which are waiting its approach. A valve opens, and the bile comes out to meet the food-a juice flows forward

CHEMISTRY OF INCIPIENT BLOOD.

317

from the pancreas, like a new saliva-and from the surface of the intestines, as it passes along, a third liquid issues to temper and chemically change it. The chyle, now milky and alkaline, is taken up by the lacteals. These minute vessels are distributed along the whole course of the intestines, extracting, at every step in its progress, new portions or constituents from the food, mixing them altogether as the vessels meet in the glandular knots, and pouring the mixture into the one common reservoir-the thoracic duct. And to insure a thorough extraction of all feeding matter, a new change takes place when the food descends into the larger intestines. It becomes acid again, and delivers to the still busy lacteals new materials with which to give the final tempering to the milky chyle as it flows towards the true bloodvessels.

All this has been explained. But it will amply repay us if we follow a little further the chemistry of this incipient blood.

Fig. 109.

Fig. 110.

Fig. 111.

O

о

109. "The human red corpuscle, showing its natural form and appearance when brought fully into focus, in which case the centre always appears light. Scattered over the field are seen one or two white corpuscles (b)."

110. "The samne seen united into rolls, as of miniature money in appearance." 111. "The blood corpuscles of the elephant, red and white, which are the largest hitherto discovered among the Mammalia." All magnified 670 times.-(From HASHasSALL's Microscopic Anatomy).

Seen under the microscope, the milky contents of the