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.
It may seem almost absurd to those unacquainted with the subject, that so much attention should be devoted to experiments on the effect of drugs on the lower animals, when our object is, as we have just stated, to ascertain their action upon human beings, and their mode of employment in the diseases of man.
But in the study of Pharmacology, just as in Histology, very much is to be learned by comparative studies. In his lectures, Ranvier admirably defines General Anatomy as Comparative Histology limited to a single organism. He illustrates this by showing that the different modes of movement which occur in some of the lower classes of the animal kingdom are to be found united in the highest. Thus leucocytes of the blood move about like amoebae. The epithelium of the respiratory passages is provided, like infusoria, with cilia; and while some muscles have the power of rapid contraction, others contract slowly, like those of some invertebrata.1
We have thus in certain parts of the bodies of the higher animals and of man, anatomical elements whose functions are performed in a way resembling that of organisms low in the scale of existence, and by examining the effects of drugs upon these low organisms we acquire knowledge which aids us in determining the action of drugs upon similar anatomical elements in the human body.
In his admirable lecture on Elemental Pathology, Sir James Paget draws attention to the distinction between the conditions of life and the essential properties of living things; and to the fact that, while the various parts of a complicated organism like the human body are closely connected together, and made to work in harmony for the common good of the organism in health, yet each part retains its own mode of life, and may sometimes develop to an excessive extent at the expense of the rest, and may destroy the organism, and itself as well. We see the power which each part possesses of carrying on individual life apart from the rest best in lower organisms or in inorganic substances, where the parts are less dependent on the welfare of the whole.
Thus, in crystals, a chip which has been broken off is replaced, and the form of the crystal restored, by putting it in a solution which will yield it the proper kind of material required. When a hydra is cut in two, each part grows into a perfect individual : a tail growing to the head part, and a head growing to the tail part. When a claw has been broken off a crab or lobster, a new one will by-and-by grow; but if the animal be divided in two, unlike the hydra it will die.
1 Legons d'anatomie generale sur le systtme musculaire, par L. Ranvier. Paris, 1880, p. 46.
As we ascend in the scale of existence the power of repair becomes less perfect. But even in the human being we see that the different parts retain their individual life, and if put into proper conditions may live, although the original body from which they were obtained were to die. Teeth, for example, which have been extracted from one person have been transplanted and grown in the jaws of another; and the transplantation of hair, skin, or of periosteum is perfectly practicable.