Here, in this present study of "pancreatic diabetes," by Dr. Vaughan Harley and others, are facts as important as any that Bernard made out: in no way contradicting his work, but adding to it. The pancreas is no longer taken to be only a sort of salivary gland out of place: over and above the secretion that it pours into the intestines, it has an "internal secretion," a constituent of the blood: it belongs not only to the digestive system, but also, like the thyroid gland and the suprarenal capsules, to the whole chemistry of the blood and the tissues. So far has physiology come, unaided by anatomy, from the fantastic notions of Lindanus and the men of his time: and has come every inch of the way by the help of experiments on animals. Professor Starling's observations, on the chemical influence of the duodenal mucous membrane on the flow of pancreatic fluid, have advanced the subject still further.
VI
THE GROWTH OF BONE
The work of du Hamel proved that the periosteum is one chief agent in the growth of bone. Before him, this great fact of physiology was unknown; for the experiments made by Anthony de Heide (1684), who studied the production of callus in the bones of frogs, were wholly useless, and serve only to show that men in his time had no clear understanding of the natural growth of bone. De Heide says of his experiments:—
"From these experiments it appears—forsan probatur—that callus is generated by extravasated blood, whose fluid particles being slowly exhaled, the residue takes the form of the bone: which process may be further advanced by deciduous halitus from the ends of the broken bone."
And Clopton Havers, in his Osteologia Nova (London, 1691), goes so far the wrong way that he attributes to the periosteum not the production of bone, but the prevention of over-production; the periosteum, he says, is put round the shaft of a bone to compress it, lest it grow too large.
Du Hamel's discovery (1739-1743) came out of a chance observation, made by John Belchier,[4] that the bones of animals fed near dye-works were stained with the dye. Belchier therefore put a bird on food mixed with madder, and found that its bones had taken up the stain. Then du Hamel studied the whole subject by a series of experiments. To estimate the advance that he gave to physiology, contrast de Heide's fanciful language with the title of one of du Hamel's papers—Quatrième Mémoire sur les Os, dans lequel on se propose de rapporter de nouvelles preuves qui établissent que les os croissent en grosseur par l'addition de couches osseuses qui tirent leur origine du périoste, comme le corps ligneux des Arbres augmente en grosseur par l'addition de couches ligneuses qui se forment dans l'écorce. Or take an example of du Hamel's method:—
"Three pigs were destined to clear up my doubts. The first, six weeks old, was fed for a month on ordinary food, with an ounce daily of madder-juice—garance grappe—put in it. At the end of the month, we stopped the juice, and fed the pig in the ordinary way for six weeks, and then killed it. The marrow of the bones was surrounded by a fairly thick layer of white bone: this was the formation of bone during the first six weeks of life, without madder. This ring of white bone was surrounded by another zone of red bone: this was the formation of bone during the administration of the madder. Finally, this red zone was covered with a fairly thick layer of white bone: this was the layer formed after the madder had been left off.... We shall have no further difficulty in understanding whence transudes the osseous juice that was thought necessary for the formation of callus and the filling-up of the wounds of the bones, now we see that it is the periosteum that fills up the wounds, or is made thick round the fractures, and afterward becomes of the consistence of cartilage, and at last acquires the hardness of bones."
These results, confirmed by Bazan (1746) and Boehmer (1751), were far beyond anything that had yet been known about the periosteum. But the growth of bone is a very complex process: the naked eye sees only the grosser changes that come with it; and du Hamel's ingenious comparison between the periosteum and the bark of trees was too simple to be exact. Therefore his work was opposed by Haller, and by Dethleef, Haller's pupil: and the great authority of Haller's name, and the difficulties lying beyond du Hamel's plain facts, brought about a long period of uncertainty. Bordenave (1756) found reasons for supporting Haller; and Fougeroux (1760) supported du Hamel. Thus men came to study the whole subject with more accuracy—the growth in length, as well as the growth in thickness; the medullary cavity, the development of bone, the nutrition and absorption of bone. Among those who took up the work were Bichat, Hunter, Troja, and Cruveilhier; and they recognised the surgical aspect of these researches in physiology. After them, the periosteal growth of bone became, as it were, a part of the principles of surgery. From this point of view of practice, issued the experiments made by Syme (1837) and Stanley (1849): which proved the importance of the epiphysial cartilages for the growth of the bones in length, and the risk of interfering with these cartilages in operations on the joints of children. Finally, with the rise of anæsthetics and of the antiseptic method, came the work of Ollier, of Lyon, whose good influence on the treatment of these cases can hardly be over-estimated.
VII
THE NERVOUS SYSTEM
As with the circulatory system, so with the nervous system, the work of Galen was centuries ahead of its time. Before him, Aristotle, who twice refers to experiments on animals, had observed the brain during life: for he says, "In no animal has the blood any feeling when it is touched, any more than the excretions; nor has the brain, or the marrow, any feeling, when it is touched": but there is reason for believing that he neither recognised the purpose of the brain, nor understood the distribution of the nerves. Galen, by the help of the experimental method, founded the physiology of the nervous system:—