XXIII. MAN, A MAMMAL

Problem XLI. A study of man as a vertebrate compared with the frog. (Laboratory Manual, Prob. XLI.)

(a) Comparison of body covering.

(b) The study of muscles.

(c) Adaptations in the skeleton.

(d) Nervous system.

Man's Place in Nature.

Although we know that man is sepa

If we

rated mentally by a wide gap from all other animals, in our study of physiology we must ask where we are to place man. attempt to classify man, we see at once he must be placed with the vertebrate animals because of his possession of a vertebral column. Evidently, too, he is a mammal, because the young are nourished by milk secreted by the mother and because his body has at least a partial covering of hair. Anatomically we find that we must place man with the apelike mammals, because of these numerous points of structural likeness. The group of mammals which includes the monkeys, apes, and man we call the primates. Although anatomically there is a greater difference between the lowest type of monkey and the highest type of ape than there is between the highest type of ape and the lowest savage, yet there is an immense mental gap.

Undoubtedly there once lived upon the earth races of men who were much lower in their mental organization than the present inhabitants.

Evolution of Man. If we follow the early history of man upon the earth, we find that at first he must have been little better than one of the lower animals. He was a nomad, wandering from place to place, living upon whatever living things he could kill with his hands. Gradually he must have learned to use weapons, and thus kill his prey, first using rough stone implements for this purpose. As man became more civilized, implements of bronze and of iron

were used. About this time the subjugation and domestication of animals began to take place. Man then began to cultivate the fields, and to have a fixed place of abode other than a cave. The beginnings of civilization were long ago, but even to-day the earth is not entirely civilized.

The Races of Man. At the present time there exist upon the earth five races or varieties of man, each very different from the other in instincts, social customs, and, to an extent, in structure. These are the Ethiopian or negro type, originating in Africa; the Malay or brown race, from the islands of the Pacific; the American Indian; the Mongolian or yellow race, including the natives of China, Japan, and the Eskimos; and, finally, the highest type of all, the Caucasians, represented by the civilized white inhabitants of Europe and America.

The Human Body a Machine. — In all animals, and the human animal is no exception, the body has been likened to a machine in that it turns over the latent or potential energy stored up in food into kinetic energy (mechanical work and heat), which is manifested when we perform work. One great difference exists between an engine and the human body. The engine uses fuel unlike the substance out of which it is made. The human body, on the other hand, uses for fuel the same substances out of which it is formed; it may, indeed, use part of its own substance for food. It must as well do more than purely mechanical work. The human organism must be so delicately adjusted to its surroundings that it will react in a ready manner to stimuli from without; it must be able to utilize its fuel (food) in the most economical manner; it must be fitted with machinery for transforming the energy received from food into various kinds of work; it must properly provide the machine with oxygen so that the fuel will be oxidized, and the products of oxidation must be carried away, as well as other waste materials which might harm the effectiveness of the machine. Most important of all, the human machine must be able to repair itself.

In order to understand better this complicated machine, the human body, let us examine the structure of its parts and thus get a better idea of the interrelation of these parts and of their functions.

Structure of the Skin. In man, the outer covering of the skin is composed of two layers. The outer part (called the epidermis) is composed largely of flattened dead cells. It is part of this layer that peels off after sunburn, or that separates from the inner part of the epidermis when a water blister is formed. The inner cells of the epidermis are provided with more or less pigment or coloring matter. It is to the varying quantity of this pigment that the light or dark complexion is due. The inmost layer of the epidermis is made up of small cells which are constantly dividing to form new cells to take the place of those in the outer layer which are lost.

The dermis, or inner layer, is largely composed of connective tissue filled with a network of blood vessels and nerves. This layer contains the sweat glands, some of the most important glands in the body. Other organs connected with the nervous system, and called the tactile corpuscles, cause this part of the skin to be sensitive to touch.

Nails and Hairs. Nails are a development from the horny layer of the epidermis. A hair is also an outgrowth of the horny layer, although it is formed in a deep pit or depression in the dermis; this pit is called the hair follicle.

HUNT. ES. BIO.-21

The Glands of the Skin. Scattered through the dermis, and usually connected with the hair follicles, are tiny oil-secreting glands, the sebaceous glands. The function of the sebaceous gland is to keep the hair and surface of the skin soft. The other glands, known as sweat glands, are to be found in profusion, over 2,500,000 being present in the skin of a normal man. These glands carry off certain wastes from the blood in the water they pass off. Thus the skin not only protects the body, but also serves as an excretory organ. Its most important function, however, is the regulation of the heat of the body. How it does this, we shall learn later. (See Chapter XXVII.)

Connective Tissue. The layer immediately beneath the dermis is known as the subcutaneous layer. It is an important storage place for fat. Underneath this layer we find a mass of flesh or muscle. Intermixed with this is a considerable amount of fat. The fat, muscle, in fact, all the tissues in the body, are held together by fibrous threads called connective tissue. Muscles and Movement. We are all aware that motion in any of the higher animals is caused by the action of the muscles. These contract to cause movement. In man and the other vertebrate animals, the muscles are almost always fastened to bones, which, acting as levers, give wide range of motion.

pe

tr

ta

sm

Muscles of the left leg of the frog; b, M. biceps; g, M. gastrocnemius; sm, M. semimembranosus; tr, M. triceps.

Arrangement of Voluntary Muscles in the Human Body.— Muscles are usually placed in pairs; one, called the extensor, serves to straighten the joint; the other, the flexor, bends the joint. Locate, by means of feeling the muscles when expanded and when contracted, the extensors and flexors in your own arm. Use the leg of a frog to determine which muscles are extensors and which flexors (see the Figure). This paired arrangement of muscles is of obvious importance, a flexor muscle balancing the action of an extensor on the other side of the joint. The end of the muscle that has the wider move

ment in a contraction is called the insertion; the part that moves least is the origin.

Microscopic Structure of Voluntary Muscle. With a sharp pair of scissors cut through a muscle at right angles to the long axis; examination will show that it is composed of a number of bundles of fibers. These fibers are held together by a sheath of connective tissue. Each of these bundles may be separated into smaller ones. If we continue this so as to separate into the smallest possible bits that can be seen with the naked eye, and then examine such a tiny portion under the compound microscope, it will present somewhat the appearance shown in the Figure. The muscle is seen to be made up of a number of tiny threads which lie side by side, held together by the sheath. Muscles, then, are bundles of long fibers. man, muscles which are under the control of the will have a striated

In

A bit of voluntary muscle fiber, showing the cross striations as seen under the microscope. (Highly magnified.)

appearance, while those which are involuntary are unstriated. Both kinds are supplied with nerves, which control them (see Figures).