INTRODUCTION 



The blood may be considered as a tissue composed of a cellular 

 part and an intercellular substance. The cellular part comprises 

 the colored and white corpuscles and the plates while the inter- 

 cellular substance consists of the fluid portion. The blood is a 

 peculiar tissue in that it is in intimate relation with practically 

 all the other tissues of the body, bringing substances to them and 

 carrying away other substances. Thus the blood is affected by all 

 the tissues. One might expect that the blood would give a good 

 deal of information as to the state of the activities of the tissues; 

 but at the present time comparatively little use is made of the 

 chemical substances contained in the plasma. In the first place 

 they are present in very small quantities, as an excess is prevented 

 by prompt elimination; and in the second place many of these 

 substances have very complicated structures. In fact the chem- 

 ical make-up of the blood is but little known. In a clinical examina- 

 tion main reliance is placed on physical and histological rather 

 than chemical methods. 



The value of an examination of the blood varies in different 

 conditions. In general it may afford sufficient information to 

 make a diagnosis in a few diseases, such as leukemia, Texas fever, 

 surra, anthrax and filariasis. It gives more or less valuable as- 

 sistance in a very large number of conditions, as secondary anemia, 

 sepsis, suppurative processes, intestinal helminthiasis and hemorr- 

 hagic diseases. Finding that the blood is normal is often a great 

 help, as it enables one to differentiate from the diseases in which 

 the blood is not normal. Hodgkin's disease is diagnosed by a 

 blood examination, yet the blood is normal in the early stages, 

 this serving to distingiush it from leukemia, which produces 

 marked changes in the blood but has otherwise similar symptoms. 

 Besides its value in diagnosis, the blood frequently gives most im- 

 portant indications as to prognosis and treatment and is of value 

 in examinations for soundness. For example, in pneumonia there 

 is ordinarily an increase in the number of leucocytes. If instead 



