THE PRACTICAL VALUE OF PURE SCIENCE 237 



between the hemispheres of the brain is a little sac about the size of 

 a pea, the pineal gland, and comparison shows that this was once a 

 third eye. Sometimes an opening persists on the side of the neck 

 below the jaw; in such a case one of the embryonic neck clefts has 

 remained open, and this in turn has relations to the gill slits of a fish. 

 All the ground plan of our bodies, the muscle cylinder within the skin, 

 next the bony scaffolding, innermost the peritoneal sack around the 

 viscera, all such relations would remain a mystery did we study only 

 the human body. But in the light of comparative anatomy and embry- 

 ology we recognize them as necessary parts of our heritage. Medicine 

 must stand upon a thorough knowledge of the structure and processes 

 of the human body, and before it can treat disorders it must under- 

 stand states of health and their origin. Comparative anatomists and 

 embryologists, the great men Harvey, Wolff, von Baer, Cuvier, Agassiz, 

 Huxlejr, Cope and Gegenbaur, such men have not only broadened the 

 field of human thought, but have also furnished the understanding of 

 the human organism. They were all pure scientists, they did not have 

 in mind the care and cure of the human body. Yet we might say 

 they accomplished more for a rational medicine than all the physicians 

 before them. How unlikely the prophecy seemed that any direct advan- 

 tage would come to mankind from the researches of Harvey, Wolff and 

 von Baer on the development of the chick, from those of Cuvier and 

 Agassiz on fossils, or from those of Huxley, Cope and Gegenbaur on 

 comparative anatomy. As the result of this change of thought we now 

 see most medical schools prescribing biological courses, and choosing 

 their professors of anatomy largely from the ranks of embryologists. 



It is hardly necessary to state that it was Louis Pasteur who laid 

 the foundation for the study of disease-producing organisms; indeed, 

 he may be said to have done more for the human race, more to prevent 

 physical misery, than any other man of the nineteenth century. He 

 had in mind, first of all, the cure, but he realized that to accomplish 

 this the mode of transmission of the disease must be understood. 

 There have followed him a long line of investigators of bacterial dis- 

 eases, and among them the purely scientific have done quite as much 

 as the purely practical. In Eussia there was a celebrated embryologist, 

 Elias Metchnikoff, who worked out the life histories of a variety of 

 animals, and was thereby led to a consideration of the part that the 

 white blood cells play in the development. This brought him to the 

 view that such cells are the guardian policemen of the body, that seek 

 out and destroy the bacteria; and this to the further idea, that health 

 is to be maintained and infection prevented by keeping the white blood 

 cells in proper numbers and activity. Metchnikoff succeeded Pasteur 

 at Paris, and though his theory of phagocytosis is far from all-sufficient, 

 it has nevertheless strongly stimulated the study of bacteriology. His 



