of heat from the body, really facilitates it, by causing, through the vascular and glandular nerve mechanisms of the skin, a greater exposure of the blood to the cooler air, and a greater quantity of moisture to be evaporated from the warm skin. When the temperature of the air reaches that of the body, the only way of disposing of the heat generated in the body is by evaporation, for radiation and conduction become impossible. In animals like man, whose cutaneous moisture is great, external heat seldom causes marked change in the rate of breathing, but in animals whose cutaneous secretion is limited, external heat distinctly affects their respiratory movements, as may be seen by the panting of a dog on a very warm day, even when the animal is at rest.

Almost more important than facilitating the escape of heat in very warm weather, are the arrangements for preventing its loss when the surroundings are unusually cold. In this case, the cold, acting as a stimulus to the vaso-constrictor nerve agencies of the skin, causes the blood to retire from surface and fill the deeper organs, where more heat is produced. This bloodless skin and the underlying fat then act as a non-conducting layer or boundary protecting the warm blood from the cooling exposure. At the same time the secretion of the sweat is controlled by a special nerve mechanism, which lessens evaporation and soon checks the secretion, thereby enabling the body to remain at the normal standard temperature.

It would then appear that the chief factors regulating the body temperature belong to the expenditure department, and may be said to be-(a) variation in the quantity of blood exposed to be cooled, and (b) variation in the quantity of moisture produced for evaporation.

These regulators have to compensate not only for differences of external temperature, but also for great fluctuations in the amount of heat produced in the tissues.

The regulating power of the skin, etc., appears to be adequate for the perfect maintenance of uniform temperature only within certain limits. When these limits are passed by the rise or fall in the surrounding medium, the preservation of a uniform tem

perature soon becomes impossible. These limits vary much in different animals, many of which have special coverings protecting them from external influences, and retain their warmth in a temperature seldom above o° C. In man the limits vary according to many circumstances, e. g., both extremes of age are more sensitive to changes of temperature. It would appear that for about 10° C. above and below the body temperature our skinregulating mechanisms are adequate, but beyond these limits external changes affect our general temperature, and if continued become injurious. Of course, by imitating with clothing the natural protection with which some animals are endowed, we can aid the normal regulating factors, and bear much greater extremes of temperature with safety or even comfort.

It is somewhat surprising that our bodies are always at the same temperature, no matter how hot or cold we feel. This is quite true, and our sensations of being hot or cold are explained as follows: When we feel hot our cutaneous vessels are full of warm blood, and this communicates to the cutaneous nerve terminals—the sensory nerves-the sensation of general warmth. On the other hand, when the cutaneous vessels are empty, the sensory nerves are directly affected by the cold of the external air. Since the full or empty state of the vessels of the skin depends generally on the heat or cold of the air, we use the expressions "it is hot or cold" and "we are hot or cold," as synonymous, because both ideas arise from the state of the skin. But we can make ourselves feel warm by violent exercise even on a frosty day, because we generate so much heat by muscular action that the cutaneous vessels have to be dilated in order to get rid of the surplus, and our skin vessels being full we feel warm. Our feelings, when we say we are warm or cold, simply depend upon our cutaneous vessels being full or empty of warm blood.

The local appreciation of differences of temperature will be discussed in the chapter dealing with the sense of Touch.

CHAPTER XXV.

CONTRACTILE TISSUES.

In the lower forms of organisms the motions executed by protoplasm suffice for all their requirements. Thus the amoeba manages to pass through its lifetime with no other kind of motion at its disposal than the flowing circulation and the budding out of its soft protoplasm. A vast number of minute organisms depend wholly upon the protoplasmic stream and the twitching of cilia for their digestive and progressive movements. Before we leave the class of animals which never pass beyond the unicellular stage, we find, however, examples in which a portion of their protoplasm is specially adapted to the performance of sudden and rapid motions. The protoplasm so modified in function deserves the name of contractile material. Thus, though the protoplasm which lies within the stalk of the bell animalcule is morphologically undifferentiated, it can contract with such rapidity that the eye cannot follow the motion.

As we ascend in the scale of animal life, the necessity for motions of various rapidity and duration at the command of the animal becomes more and more urgent, and so we find not only one, but several kinds of tissue specially adapted for carrying out motions of different rate and duration.

As a general rule, the more rapid the contraction it performs the more the tissue differs from the original type of protoplasm; and the slower and more persistent the contraction, the more the tissue elements resemble protoplasmic cells. Thus, in the minute blood vessels, as we have seen, a very prolonged form of contraction, only varied by partial relaxations, is the rule, and gives rise to the tone of the arterioles, and the contractile elements differ but little from ordinary protoplasmic cells. The intestinal movements are rapid compared with those of the arterial muscles, and in them we find a thin, elongated form of muscle cell. In the heart a forcible and quick contraction takes place, which, how

ever, is slow when compared with the sudden jerk of a single spasm of a skeletal muscle, and its texture is different, being a form intermediate between the slow-contract

A

FIG. 177.

ing smooth muscle and the quick-contracting striated skeletal muscle.

By borrowing examples from the lower animals, this parallelism of structural differentiation and increase of functional energy can be more perfectly demonstrated, and we can make out a gradual scale of increasingly rapid motion corresponding with greater complexity of structure.

HISTOLOGY OF MUSCLE.

The term muscle includes the textures in which the protoplasm is specially differentiated for purposes of contraction.

The muscle tissues of the higher animals may be divided into two classes: (1) non-striated or smooth, and (2) striated, in which again there are some slight variations.

The non-striated muscle tissue is that in which the elements are most like contractile protoplasmic cells, and have so far retained the typical form as to be easily recognizable as cells when separated one from the other. These cells are more or less elongated, flattened, homogeneous elements with a single, long, rod-shaped nucleus and no cell wall. They are tightly cemented together by a tough elastic substance, so that their tapering extremities fit closely together and form commonly a dense mass or Muscle cells, showing sheet. Sometimes they branch more or less the protoplasm of the regularly, and then are arranged in netcell and nucleus. works.

different condition of

These cells vary greatly in size as well as in the relation of their length to their width, in some places deserving the

name fibres, or fibre cells, and in others being only elongated cells.

The striated muscle tissue is that of which the skeletal muscles and the heart are composed. It therefore forms the larger proportion of the animal, known as flesh. The flesh can, by judicious dissection, easily be divided into single parts called muscles, each of which contains many other tissues, and is so attached as to carry on certain movements, and may, therefore, be regarded as an organ.

Such a muscle is enclosed in a sheath of connective tissue, for which sheet-like' partitions or septa pass into the mass of the muscle and divide it into bundles of fibres, which they enclose. These septa also act as the

bed in which the vessels and nerves lie.

The tissue of the heart differs from the striated muscle in being made up of truncated, oblong branching cells with a central nucleus and no sarcolemma (see page 262).

The bundles of fibres of skeletal muscle vary much in size, giving a coarse or fine grain in different muscles; they are composed of a greater or less number of fibres, which, lying side by side, run parallel one to the other. The single fibres of striated muscle vary in length, sometimes reaching 4-5 cm. (2 inches), but being on an average much shorter,

FIG. 178.

Short striated cells of the heart muscles, separated one showing the truncated (a), or divided (c), ends and branches (b).

they only extend the entire length of a muscle in the case of very short muscles. In long muscles their tapering points are made to correspond with those of other fibres to which they are firmly attached. The soft fibres are pressed by juxtaposition into prismatic forms, so that in a fresh condition they appear polygonal in transverse section. When freed from all pressure or traction they become cylindrical, and the transverse striation