presented by a dog, the most obvious fact is his power of moving from place to place, a power produced by the interplay of muscles and bones. We observe, also, that his motions are neither mechanical nor irregular; there is method in his movement. He has the power of willing, seeing, hearing, feeling, etc.; and these functions are accomplished by a delicate apparatus of

nerves.

But the dog does not exhibit perpetual motion. Sooner or later he becomes exhausted, and rest is necessary. Sleep gives only temporary relief. In every exercise of the muscles and nerves there is a consumption or waste of their substance. The blood restores the organs, but in time the blood itself needs renewal. If not renewed, the animal becomes emaciated, for the whole body is laid under contribution to furnish a supply. Hence the feelings of hunger and thirst, impelling the creature to seek food. Only this will maintain the balance between waste and repair. We notice, therefore, an entirely different set of functions, involving, however, the use of motion and will. The dog seizes a piece of meat, grinds it between his teeth and swallows it. It passes into the stomach, where it is digested, and then into the intestine, where it is further changed; there the nourishing part is absorbed, and carried to the heart, which propels it through tubes, called blood vessels, all over the body. In this process of digestion, certain fluids are required, as saliva, gastric juice, and bile: these are secreted by special organs, called glands. Moreover, since not all the food eaten is fitted to make blood, and as the blood itself, in going around the body, acts like a scavenger, picking up worn-out particles, we have another function, that of excretion, or removal of useless matter from the system. The kidneys and lungs

do much of this; but the lungs do something else. They expose the blood to the air, and introduce oxygen, which, we shall find, is essential to the life of every animal.

These centripetal and centrifugal movements in the body throwing in and throwing out - are so related and involved, especially in the lower forms, that they can not be sharply defined and classified. It has been said that every dog has two lives, — a vegetative and an animal. The former includes the processes of digestion, circulation, respiration, secretion, etc., which are common to all life; while the functions included by the latter, as motion, sensation, and will, are characteristic of animals. The heart is the center of the vegetative life, and the brain is the center of the animal life. The aim of the vegetative organs is to nourish the individual, and reproduce its kind; the organs of locomotion and sense establish relations between the individual and the world without. The former maintain life; the others express it. The former develop, and afterward sustain, the latter. The vegetative organs, however, are not independent of the animal; for without muscles and nerves we could not procure, masticate, and digest food. The closer the connection and dependence between these two sets of organs, the higher the rank.78

All the apparatus and phenomena of life may be included under the heads of

NUTRITION,
MOTION,

SENSATION,

REPRODUCTION.

These four are possessed by all animals, but in a variety of ways. No two species have exactly the same mechanism and method of life. We must learn to dis

tinguish between what is necessary and what is only accessory. That only is essential to life which is common to all forms of life. Our brains, stomachs, livers, hands, and feet are luxuries. They are necessary to make us human, but not living, beings. Half of our body is taken up with a complicated system of digestion; but the amoeba has neither mouth nor stomach. We have an elaborate apparatus of motion; the adult oyster can not stir an inch.

Nutrition, Motion, and Sensation indicate three steps up the grade of life. Thus, the first is the prominent function in the coral, which simply "vegetates," the powers of moving and feeling being very feeble. In the higher insect, as the bee, there is great activity with simple organs of nutrition. In the still higher mammal, as man, there is less power of locomotion, though the most perfect nutritive system; but both functions are subordinate to sensation, which is the crowning development.

In studying the comparative anatomy and physiology of the animal kingdom, our plan will be to trace the various organs and functions, from their simplest expression upward to the highest complexity. Thus Nutrition will begin with absorption, which is the simplest method of taking food; going higher, we find digestion, but in no particular spot in the body; next, we see it confined to a tube; then to a tube with a sac, or stomach; and, finally, we reach the complex arrangement of the higher animals.

CHAPTER IX

NUTRITION

Nutrition is the earliest and most constant of vital operations. So prominent is the nutritive apparatus, that an animal has been likened to a moving sac, organized to convert foreign matter into its own likeness, to which the complex organs of animal life are but auxiliaries. Thus, the bones and muscles are levers and cords to carry the body about, while the nervous system directs its motions in quest of food.

The objects of nutrition are growth, repair, propagation, and supplying energy to perform the work, or functions, of the body. The first object of life is to grow, for no animal is born finished. Some animals, like plants, grow as long as they live; but the majority soon attain a fixed size. In all animals, however, without exception, food is wanted for another purpose than growth, namely, to repair the waste which is constantly going on. For every exercise of the muscles and nerves involves the death and decay of those tissues, as shown by the excretions. The amount of matter expelled from the body, and the amount of nourishment needed to make good the loss, increase with the activity of the animal. The supply must equal the demand, in order to maintain the life of the individual; and as an animal can not make food, it must seek it from without. Not only the muscles and nerves are wasted by use, but every organ in the body; so that the whole structure needs constant renewal. An

animal begins to die the moment it begins to live. The function of nutrition, therefore, is constructive, while motion and sensation are destructive.

Another source of demand for food is the production of germs, to propagate the race, and the nourishment of such offspring in the egg and infantile state. This reproduction and development of parts which can maintain an independent existence is a vegetative phenomenon (for plants have it), and is a part of the general process of nutrition. But it will be more convenient to consider it hereafter (Chapters XXII., XXIII.). Still another necessity for aliment among the higher animals is the maintenance of bodily heat. This will be treated under the head of Respiration.

For the present, we will study nutrition, as manifested in maintaining the life of an adult individual.

In all animals, this process essentially consists in the introduction of food, its conversion into tissue, its oxidation, and the removal of worn-out material.

I. The food must be procured, and swallowed. (Ingestion.)

2. The food must be dissolved.

(Digestion.)

3. The nutritive fluid must be taken up, and then distributed all over the body. (Absorption and circulation.)

4. The tissues must repair their parts wasted by use, by transforming a portion of the blood into living matter like themselves. (Assimilation.)

5. Certain matters must be eliminated from the blood, some to serve a purpose, others to be cast out of the system. (Secretion and excretion.)

6. In order to produce work and heat, the food must be oxidized, either in the blood or in the tissues, after assimilation. The necessary oxygen is obtained through exposure of the blood to the air in the lungs. (Respiration in part)