The difference between these classes of remedies is simply one of degree. When a drug increases the secretion of sweat only slightly, so that it can still evaporate from the skin without running down in drops, it is called a diaphoretic; but when it increases it so greatly chat it can no longer evaporate, and streams down the skin, it is called a sudorific.

The secretion of sweat, like that of saliva, consists in the formation of the secretion by the cells of the gland from the material which is yielded by the fluid in the lymph-spaces around the gland.

New material is constantly supplied to this fluid by the blood which circulates in the vessels. We therefore find that increased circulation of blood through the cutaneous vessels and increased secretion of sweat usually accompany one another, but this is not always the case. In the sweat-glands, as in the salivary glands, the secreting nerves which regulate the activity of the cells are independent of the vascular nerves which regulate the capacity of the vessels. In fever or in poisoning by atropine the vessels may be widely dilated and the current of blood through them rapid, while the secretion of sweat is arrested. On the other hand, in dying persons we see a copious secretion of sweat occur, while the circulation through the skin has become very feeble or almost stagnant. A certain amount of sweat, indeed, may even be secreted by amputated limbs, the material for it being afforded by the lymph around the glands. But profuse secretion of sweat cannot go on long unless the gland is freely supplied with blood, for otherwise the supply of new material would cease. Dilatation of the vessels therefore aids the secretion of sweat. Dilatation may be induced by section of vaso-motor nerves or stimulation of vaso-dilating nerves. Thus, when the sympathetic is cut in the neck of a horse, dilatation of the vessels is produced by the section, and sweating occurs on that side.

The vaso-dilating and secreting nerves of the sweat-glands usually run together, and by irritation of a nerve-trunk, such as that of the sciatic, the vessels of the foot may be dilated, and sweating excited.

Warmth usually increases both the circulation of blood in the skin and the secretion of sweat; while cold has the contrary effect.

The nerve-centres which excite the secretion of sweat appear to be situated in the spinal cord; the centre for the posterior extremities being situated in the upper lumbar and lower thoracic part of the cord in the cat; while that for the upper extremities in the same animal is situated in the lower part of the cervical region of the cord.

The sweat-glands may be excited to secrete:

(1) By the action of drugs upon the terminations of nerves in the glands.

(2) By the action of drugs on the sweat-centres themselves.

(3) Reflexly by stimulation of sensory nerves.

(4) By mental stimuli.

An example of the stimulation of sweating by the action of drugs on the nervous terminations in the glands themselves is afforded by pilocarpine, which will cause secretion even when the nerves which connect the centres with the glands have been cut.

Secretion may be also arrested by the paralysing action of drugs upon the terminal fibres; thus, atropine, locally injected, prevents the secretion of sweat, however much the nerve going to the gland or the nerve-centres be stimulated; and atropine also antagonises the effect of pilocarpine on the nervous terminations, and arrests the secretion which the latter causes.

The nerve-centres may be stimulated directly by the condition of the blood which is passing through them, or reflexly by irritation of sensory nerves. Stimulants of these nerve-centres are : (1) a venous condition of the blood; (2) high temperature of the blood; and (3) poisons, especially nicotine.

A venous condition of the blood is one of the most powerful stimulants, and it is to this that the sweats which precede death are in all probability due; for while watching a patient dying, I have observed that drops of sweat appeared on the brow just at the time that the blood became venous, as was evidenced by the commencing lividity of the finger-nails and lobes of the ears. Under such conditions, while the secreting cells are strongly stimulated, the circulation is very feeble.

A high temperature is also a powerful stimulant. In considering its action we must take into account the effect of the warm blood upon the sweat-centres in the cord, as it circulates through them, and its local action also on the sweat-glands themselves. Up to a certain point it appears to have the effect of dilating vessels and of increasing the activity of the glands by acting both on the sweat-centres and on the periphery.

Local warmth to one foot increases the secretion of sweat, and local cold diminishes it in that foot, when the glands in all four feet of an animal are stimulated equally either by excitement of the sweat-centres or by the action of pilocarpine on the peripheral ends of the sweat-nerves.1

1 Luchsinger, Pfluger's Archiv, 1876, vol. xviii. p. 480.

The sweat-centres appear to be directly stimulated by nicotine, but the action of this drug may be partly due also to a reflex effect on those centres through the nerves of the stomach.

The sweat-centres appear to be reflexly excited by severe irritation of any sensory nerve passing from the surface of the body, and the point at which the irritation is applied does not seem to be of much importance. They are probably stimulated reflexly from the stomach, as in the sweating which accompanies nausea.

The power of the brain to stimulate the sweat-centres is shown in the effect of mental emotion, and direct irritation of the medulla oblongata will cause sweating in cats even some time after death.

Excretion by the Sweat-glands. - A number of substances taken into the body pass out in small quantities through the skin. Aromatic and volatile substances appear to pass readily, so also benzoic acid, hippuric and cinnamic acid, tartaric acid, succinic acid, iodide of potassium, quinine, corrosive sublimate, arseniates of sodium and potassium. When arseniate of iron has been taken, curiously enough, arsenious acid has been found in the sweat, and iron in the urine. Some colouring matters are excreted especially by the skin of the armpits, and the underclothing may sometimes be found stained of a brick-red colour at these parts. I have observed this in some cases after drinking claret or port, but it only occurs exceptionally after the employment of these wines, and it is possible that it is due to adulteration with foreign colouring matters, for I have also noticed it in cases where no wine has been drunk, but where pickled red cabbage or beetroot has been eaten.