CHAPTER VIII. 



BLOOD-VESSELS LYMPHATIC VESSELS. 

 BLOOD-VESSELS. 



BLOOD-VESSELS are of three kinds : arteries, veins, 

 and capillaries. Although merging without sharp 

 demarcation into one another, these vessels, in their 

 typical forms, present distinct differences in struc- 

 ture. The capillaries being the simplest, it will be 

 convenient to commence with them. 



If we examine a capillary vessel, either fresh or 

 after it has been in preserving fluids, it presents 

 the appearance of a narrow tube, with very thin, 

 homogeneous walls, in which, at frequent intervals, 

 elongated nuclei are imbedded, their long axes 

 being parallel with the axes of the tube. If, how- 

 ever, we inject the vessels with a dilute solution of 

 nitrate of silver, and expose them to the light, we 

 find that the inside of the tube is divided by narrow 

 black lines into elongated, irregular-shaped spaces ; 

 and if we then stain the specimen with hrematoxy- 

 iin, we find that a nucleus lies in each space. The 

 walls of the capillaries are, then, not formed by a 

 homogeneous membrane, but made up of cells having 



