It has been elsewhere explained that the heart is a muscular organ which, by its contractions, propels the blood over the body. If it contracts with inadequate force the cavities are not properly emptied, nor the bloodvessels filled, and nutrition is impaired. If by shock or haemorrhage an insufficient blood-supply is sent to the brain, its functions are suspended, and the animal faints and falls. The recumbent position thus assumed is favourable to recovery, and with removal of the cause the heart's action may be restored. When syncope threatens a horse he staggers, and in trying to keep upon his feet gives us some warning of his condition, which it is well to heed for his sake as well as our own safety. Trembling, sighing, and staggering will be noticed, and an examination of the membranes shows them to be blanched, or pallid, like the countenance of a person about to swoon. If a heart stimulant is given, the function of the organ will probably be restored for the time, and the brain again recovering its due supply of blood, consciousness will return.
Any failure of the heart acts upon its own substance, which, like every other part of the body, derives its nourishment from the blood.
The heart receives its stimulus through the nervous system, and it has within it certain minute nervous bodies (ganglia) designed to excite and carry on the rhythmic movements which are commonly called "beats". Remedies which influence the beats of the heart do so through these nerve-centres being acted upon by agents which have entered the blood-stream. They are further controlled by the pneumogastric nerve, which slows the action of the organ, and by the sympathetic nerves, which excite it; and it is through the medium of these that stimulants are able to increase both the frequency and the force of the muscular contractions.
Heat is a rapid stimulant when applied to the walls of the chest, or as a hot fluid passed into the stomach; but in the equine patient we have to rely chiefly upon drugs, and of these alcohol, ammonia, ether, and camphor are the chief.
Ammonia is a gas dissolved in water, but in that condition is not often employed in horse practice.
The chief preparations are: -
Strong Liquid Ammonia. Dilute Liquid Ammonia. Aromatic Spirit of Ammonia. Liniment of Ammonia.
Ammonia is a stimulant when applied to the skin, the neutral salts having the least action and the strong liquid the greatest, the latter producing vesicles like an ordinary blister. Weak solutions are used to neutralize the stings of insects, which are usually acid. When inhaled, ammonia rouses the heart and quickens the pulse and respiration. It is employed in this way as a restorative after operations under chloroform when the patient is disposed to remain too long under its influence. The liquid is particularly irritating to the nostrils, and should not be brought into actual contact either with the thin skin covering them or the membrane within.
Ammonia has a stimulating effect also when passed into the stomach, and the preparations to be preferred for internal administration are the aromatic spirit (sal volatile), largely diluted, or a solution of the carbonate. In the latter form it not only increases the heart's action, but stimulates the stomach and bowels before it enters the circulation. For this reason it is a valuable remedy in flatulent colic, and for various forms of indigestion it is prescribed with vegetable tonics.
It is given in respiratory affections because it stimulates the membranes of the bronchial tubes to expel mucous matter from their surface. The acetate is employed as a febrifuge, and promotes the action of the skin.
Chloride of ammonium is given in chronic diseases of the liver, and as an external remedy it is used in combination with other drugs as a cooling or evaporating lotion for inflamed joints and tendons.