These are remedies which reduce the temperature of the body in fever. They act much more powerfully when the temperature is abnormally high than when it is normal.

The constant temperature of warm-blooded animals depends upon the maintenance of a proper balance between the amount of heat generated in the body, chiefly by oxidation, and the amount given off to the surrounding medium - air or water. The heat is chiefly generated in the muscles and glands. It is chiefly given off by the skin, although some is also lost by the lungs, etc.

A little heat, but not much, may be given off by radiation alone. The power of dry air to take up heat is very slight, and so the skin is not much cooled, and very little sensation of cold is felt at temperatures much below 0° if the air is both still and dry. If the air be moist its capacity for heat is much greater, and the loss of heat from the skin being much more rapid, a person may actually feel the weather colder at 4° F. than at - 40° F., the air being still in both cases. If air, either dry or moist, is in motion, so that fresh portions of it come successively into contact with the skin, the loss of heat is much more rapid, and a little wind will render even dry air unbearably cold at a temperature which would be quite supportable if the air were still.

Loss of heat occurs more readily in small animals than in large. This is represented diagrammatically in Fig. 144.

It is to be observed that during sleep the action of the vasomotor centre is less, the vessels of the surface dilate, and loss of heat, with danger of consequent chill, takes place more rapidly. For the effects of local chill to the surface, the results of Ross-hach's experiments may be consulted (p. 251).

Fig. 144.   Diagram to show that loss of heat occurs mure readily in small animals than in large. The unshaded part in a and b represents the surface through which heat is lost; the black part shows the heat producing part of the body.

Fig. 144. - Diagram to show that loss of heat occurs mure readily in small animals than in large. The unshaded part in a and b represents the surface through which heat is lost; the black part shows the heat-producing part of the body. These are shown separately in a' and V, from which it is evident that in the small animal the heat-producing area is about the same size as, while in the large animal it is double the size, of the heat-dispersing area.

But heat may be generated in muscles and glands apart from the circulation in them, and Sachs and Aronsohn have shown that a centre regulating the production of heat is situated in the neighbourhood of the corpora striata.1 It is probable that the temperature may be affected by drugs acting on the nervous system apart from the circulation and also by drugs which affect the tissues themselves (p. 58 et seq.).1

Fig. 145.   Diagram to illustrate the action of alterations in the circulation of the surface of the body and the internal organs and muscles upon temperature.

Fig. 145. - Diagram to illustrate the action of alterations in the circulation of the surface of the body and the internal organs and muscles upon temperature. In this figure the superficial vessels are represented as contracted, and there is therefore not only less loss of heat, but the blood being driven to the internal organs and muscles, the circulation in them is increased and the production of heat augmented. The parts where heat is produced are the dark, the darkness being in proportion to the greater production. The parts where heat is retained without much being formed, e.g. the blood, are moderately shaded. Those where heat is lost are left white. In the intestine heat is both formed and lost (p. 418), and so the intestines are partly dark and partly light.

1 Deutsche med. Wochenschr., December 1884.

The circulation exercises a very important influence upon (1) the amount of heat lost from the surface and (2) the amount of heat produced in the internal organs and muscles. This is represented diagrammatically in Figs. 145 and 146.

Fig. 146.   Diagram to illustrate the action of alterations in the circulation of the surface of the body and the internal organs and muscles upon the temperature.

Fig. 146. - Diagram to illustrate the action of alterations in the circulation of the surface of the body and the internal organs and muscles upon the temperature. In the diagram the cutaneous vessels are represented as dilated, and thus not only is more heat lost from the surface, but, blood being withdrawn from the internal organs and muscles, the circulation in them is lessened and less heat produced.

The vessels of the skin form a cooling apparatus (p. 440), while heat is generated in the muscles, voluntary and involuntary, and in glands, e.g. the intestine and liver. The intestine is, however, only protected by the thin abdominal walls from the cooling action of the external air, and so it may act either in cooling or warming the body, according to circumstances.

When the vessels of the surface are dilated, the blood not only courses freely through them and becomes cooled, but, being withdrawn from the muscles and glands, there is less heat produced. The reverse is the case when the cutaneous vessels are contracted. The condition of the vessels depends on the action of the vaso-motor centre, and drugs acting upon it may greatly modify the temperature.

Antipyretics may be divided into two great classes : those which lessen the production of heat, and those which increase the loss of heat; and these again may be subdivided as shown in the following table :1 Umbach, Archiv f. exp. Path. u. Pharm. 1886, xxi.