IN MEDICINE. 19 



Having glanced at the circulatory apparatus, we will now see 

 what the microscope has taught us about the nutritive fluid, the 

 blood. We find that, instead of being — as it appears to the 

 naked eye — a red fluid, quite clear and homogeneous, in reality 

 it consists of a nearly colourless fluid, in which are suspended 

 numerous cells, or corpuscles, of two kinds. Some are circular, 

 flattened discs — about 1-3 200th of an inch in diameter and 

 about one-fourth of that thickness, having a yellowish red tinge, 

 the aggregation of which gives the red colour to the blood. The 

 other kind of cells are colourless, larger, and much less numerous 

 than the red ones, having a diameter of about i-25ooth of an 

 inch, and differing from them by the irregularity of their form, and 

 also by the fact that they have a curious trick of constantly 

 changing their shape. If we carefully watch one under the 

 microscope, with a magnifying power of five or six hundred 

 diameters, we see that every part of the surface is constantly 

 changing, undergoing active contraction, or being passively diluted 

 by the contraction of other parts. This power of independent 

 contractility is similar to what we see taking place in those sim- 

 plest organisms which are met \^^th in stagnant water, and are 

 called Amcebce^ and this movement has therefore been called the 

 Amoeboid movement of the white cells. 



In a similar manner, by the same instrument, we have learned 

 the minute structure of the brain, the general distribution of nerve- 

 fibres to every organ, the minute structure of muscular fibre, the 

 structure of various glands, the marvellous adaptation of the skin 

 as a covering to the body, with its hairs, perspiration-glands, and 

 other appendages. 



But we must leave this first part of our subject, and proceed 

 to look at some of the teachings of the microscope in Pathology. 



As in Physiology, it has taught us the minute structure of 

 every tissue of the body in health ; so from it we learn the struc- 

 tural alterations which take place in disease, and from these 

 alterations we try to discover the causes of the morbid action and 

 to find out the means by which they can be removed. 



Thus, by means of it we can watch the very changes going on 

 in the blood-vessels during the process of inflammation. Putting 

 a few grains of mustard on the web of a frog's foot, we see, in the 



