CHAPTER VII 



THE FROG (iVntilllied) : THE ^^CROSCOPICAL EXAMINATION 

 OF THE SIMPLE TISSUES. 



Before carrying our enquiries any further into the 

 anatomy and physiology of the frog it will be necessary to 

 devote some consideration to its microscopic structure or 

 histology, since there are many matters in connection with 

 the various organs which can be further elucidated only by 

 the examination of the minute structure of the organs as 

 revealed by the microscope {see p. 119). 



Let us, first of all, examine a drop of the frog's blood 

 under the low power of the microscope. It will at once be 

 seen that the blood is not a simple homogeneous fluid, but 

 that it contains a large number of minute solid bodies 

 floating in it. These are called by the general name of 

 blood-corpuscles : the fluid part of the blood in which they 

 float is called the plasma. At first, owing to currents in the 

 fluid, the corpuscles will be found to move to and fro, but 

 after a time they come to rest. Under the high power you 

 will notice that the corpuscles are of two kinds. The greater 

 number of them are regularly oval in form (Fig. 26, C), and 

 of a yellow colour. If the drop of blood is thick enough 

 in one part for the corpuscles to lie over one another, so 



