660 PHYSIOLOGICAL CHEMISTRY. 



2. Evaporate a drop of the fluid on a microscope slide with a 

 minute fragment of sodium chloride, cover with a cover-glass, allow 

 a drop of glacial acetic acid to enter from the side and warm gently ; 

 abundant crops of hsemin crystals are seen under the microscope 

 after cooling. 



3. Add a drop of the fluid to some freshly prepared tincture of 

 guaiacum in a test-tube and float on the surface of an ethereal solu- 

 tion of hydrogen dioxide ; a blue ring forms at the junction of the 

 ethereal solution and the guaiacum. (Blood is, however, not the only 

 substance showing this reaction. 



4. The spectroscope shows bands characteristic of haemoglobin. 



5. The biologic blood-test. This test depends upon the fact that 

 animals (rabbits) injected with human blood-serum will develop a 

 specific antibody. This antibody is present in the blood-serum of 

 the injected animal and is termed a " precipitin," because it produces 

 a visible precipitate when the serum of the animal is mixed with a 

 solution of human blood-serum. As the technique is very intricate, 

 and the results are worthless unless carefully controlled, no details 

 can be given here. Positive results can be obtained with blood or 

 blood-stains many years old. Human protein of other origin (milk, 

 semen, etc.) and protein from the higher anthropoid apes will also 

 give positive results. 



The immune bodies of the blood-serum. When foreign protein 

 is introduced into an animal by injection or, as in disease, by infection 

 with bacteria, it is found that there is a response on the part of the 

 animal which causes the presence of certain substances (immune 

 bodies, antibodies) in the circulating blood. These have a specific 

 action upon the foreign protein. To this class belong agglutinins, 

 lysins, etc. Agglutinins are capable of causing an agglutination or 

 clumping of the corresponding bacteria. They have proved of great 

 clinical value in the diagnosis of typhoid fever by means of the Widal 

 reaction. This test consists in observing microscopically or macro- 

 scopically a mixture of active typhoid bacilli and the blood-serum of 

 the patient. If agglutinins are present, and the bacteria are clumped 

 within a certain time by a certain dilution of serum, it is shown that 

 the patient has, or in some cases has had, typhoid fever. Bacterio- 

 lysins are antibodies capable of dissolving the corresponding bacteria. 

 Opsonins are capable of producing some change in bacteria, whereby 

 it becomes possible for the leucocytes of the blood to ingest them 

 (phagocytosis). They are normally present in varying amounts. It 

 is manifest that these bodies form a part of the defensive mechanism 



