This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Some people object entirely to experiments upon animals. They do this chiefly on two grounds. The first is that such experiments are useless, and the second is that, even if they were useful, we have no right to inflict pain upon animals.
The first objection is due to ignorance. Almost all our exact knowledge of the action of drugs on the various organs of the body, as well as the physiological functions of these organisms themselves, has been obtained by experiments on animals.
The second objection is one which, if pushed to its utmost limits and steadily carried out, would soon drive man off the face of the earth.
The struggle for existence is constantly going on, not only between man and man, but between man, the lower animals and plants, and man's very being depends upon his success.
We kill animals for food. We destroy them when they are dangerous like the tiger or cobra, or destructive like the rat or mouse. We oblige them to work for us, for no reward but their food; and we urge them on by whip and spur when they are unwilling or flag. No one would think of blaming the messenger who should apply whip and spur to bring a reprieve, and thus save the life of a human being about to die on the scaffold, even although his horse should die under him at the end of the journey. Humane people will give an extra shilling to a cabman in order that they may catch the train which will take them to soothe the dying moments of a friend, without regarding the consequences to the cab-horse. Yet if one-tenth of the suffering which the horse has to endure in either of the cases just mentioned were to be inflicted by a physiologist in order to obtain the knowledge which would help to relieve the suffering and lengthen the life, not of one human being only, but of thousands, many persons would exclaim against him. Such objections as these are due either to want of knowledge or want of thought on the part of the people who make them. They either do not know the benefits which medicine derives from experiment, or they thoughtlessly (sometimes, perhaps, wilfully) ignore the evidence regarding the utility of experiment.
One of the most important objections that has been raised to this mode of experiment is that the action of drugs on the lower animals is quite different from their action on man. This objection has a certain amount of truth, but is in the main groundless. The action of drugs on man differs from that on the lower animals chiefly in respect to the brain, which is so much more greatly developed in man.
Where the structure of an organ or tissue is nearly the same in man and in the lower animals, the action of drugs upon it is similar. Thus we find that carbonic oxide and nitrites produce similar changes in the blood of frogs, dogs, and man, that curare paralyses the motor nerves alike in them all, and veratrine exerts upon the muscles of each its peculiar stimulant and paralysing action.
Where differences exist in the structure of the various organs, we find, as we would naturally expect, differences in their reaction to drugs. Thus the heart of the frog is simpler than that of dogs or men, and less affected by the central nervous system. We consequently find that while such a drug as digitalis has a somewhat similar action upon the hearts of frogs, dogs, and men, there are certain differences between its effect upon the heart of a frog and that of mammals. In all it seems to affect the muscular substance and cause increased contraction. But while the frog almost invariably dies with the heart in a state of tetanic contraction, this is not the case with dogs or men, where the heart sometimes is found in diastole after death.
Ipecacuanha or tartar emetic will cause vomiting in man, but does not do so in rabbits. The reason of this is that the position of the stomach in the rabbit is different from that in man, and is such that the animal cannot vomit. In dogs, however, the position of the stomach agrees with that of man, and tartar emetic or ipecacuanha causes vomiting in both. Belladonna offers another example of apparent difference in action - a considerable dose of belladonna will produce almost no apparent effect upon a rabbit, while a smaller dose in a dog or a man would cause the rapidity of the pulse to be nearly doubled. Yet in all three - rabbits, dogs, and men - belladonna paralyses the power of the vagus over the heart. The difference is, that in rabbits the vagus normally exerts but little action on the heart, and the effect of its paralysis is consequently slight or hardly appreciable, the pulse being normally almost as quick as it is after the vagus is paralysed. In dogs and men, on the contrary, the vagus is constantly exerting considerable restraining power over the heart, and the effects of its paralysis at once attract attention.
An example of the apparent difference in the effect of a drug on different animals is afforded by nitrite of amyl. If we measure the pressure of the blood in the arteries of a rabbit and of a dog, and then cause them to inhale nitrite of amyl, we find that the small vessels have become widened and allow the blood to pass easily out of the arterial system into the veins, so that the pressure sinks considerably in the rabbit, whereas it sinks only slightly in the dog. The action seems at first sight different; but when we examine it more closely, we find that the heart of the dog is no longer beating slowly, but very quickly, so as to keep up the pressure, notwithstanding the rapid flow of the blood through the widened vessels, while the heart of the rabbit was going so fast before that it could not go much more quickly. If we cut the vagi in the dog, so that the heart goes as quickly as in the rabbit before it begins to inhale, the blood-pressure sinks during the inhalation, just as it does in the rabbit.1
One of the most marked differences between the action of a drug upon lower animals and upon man is to be found in the effect of morphine upon frogs and upon pigeons. In frogs it causes convulsions; on pigeons, even in large doses, it produces no apparent effect. But although its effects are not appreciable to the eye, they exist nevertheless, and on applying the thermometer it is found that morphine lowers the temperature of pigeons many degrees. On comparing the effect of the drug on frogs with its effect on man, we see that in the frog the cerebral hemispheres are very slightly developed indeed as compared with man, and in the latter the effects of the drug upon the spinal cord are usually completely concealed by the narcotic effect of the drug upon the brain. In children, however, and in some races of man where the cerebral hemispheres are less developed than in Europeans, the convulsant action of morphine manifests itself. Occasionally we find individuals who are almost proof against the action of morphine, and who take large doses of it without any apparent effect. Whether in these persons it lowers the temperature as it does in pigeons is a point which remains to be ascertained.
By means of experiments upon animals, then, we are able to ascertain the action of drugs upon those organs of the body which are alike in man and animals; and the very differences which exist between the various sorts of animals, help us to understand the action of drugs more thoroughly.