Page:Popular Science Monthly Volume 69.djvu/19

Rh from our human point of view at the wrong time and in the wrong place, but which are not of themselves wrong or diseased, though they become so in man's nomenclature by their mode of occurrence. This broader conception of degeneration affords a new foundation for further investigation, and by the hearty cooperation of the embryologist and the pathologist we may expect new enlightenment.

The fourth head under which we classed the work of embryologist corresponded to the field of anatomical research. We all know that the embryological explanation of the anatomical disposition in the adult is a real and clarifying explanation. Certainly no teacher of anatomy to-day, competent to his work, will undertake to teach the structure of the brain, of the urogenital system or of the heart, except on an embryological basis. But there are a great many other anatomical conceptions which may best be made clear if we start with an examination of the conditions in the embryo. My experience as a teacher has afforded me many examples of this. Let me mention a few. The arrangement of the great cephalic nerves is a subject of peculiar difficulty to the student, but by the examination of a few properly chosen sections through the head of mammalian embryos all the essential topographical relations can be made easily understandable, and these essential relations are never obliterated by any further development. The disposition of the peritoneum is one of the greatest bugbears to the first-year medical student. But let him study the peritoneum in its relation to the viscera in the young embryo and he easily overcomes his difficulties and gets a clear and correct conception of the topographical relations of the peritoneal membrane and is able thereafter to comprehend the secondary modifications by which the adult topography is so much complicated. So too in regard to the thorax, a few sections from embryos give definite and exact conceptions of the fundamental relations of the heart and lungs, the mediastinum, and of the pleural and pericardial membranes. A good student may obtain from such a section a visual image which he will carry with him throughout life and which will always serve to make clear in his mind all these anatomical relations. One more similar instance may suffice. Students are always perplexed by the nature and mutual connections of the three membranes of the spinal cord and brain. Here also experience convinces us that sections of embryos reveal the facts so perfectly that they are readily comprehended and not easily forgotten. But I think I need not argue further to convince you that embryology as an aid to anatomical study is of incalculable value, and ought, if we are to do our anatomical teaching conscientiously, to be included in every medical curriculum.

Not infrequently the study of embryos establishes entirely new anatomical conceptions. An instance of this is offered by the study of blood vessels. We have learned in recent years that in addition to