68 



AN INTRODUCTION TO ZOOLOGY 



thickness of the coats. The tunica adventitia is thicker, but con- 

 tains a smaller number of elastic fibres. The tunica media and 

 tunica interna are very much thinner, especially the former, and 

 neither has so much elastic tissue. The result is that the wall of 

 the vein is thinner and inelastic, so that when empty and cut it 



collapses. As an artery and a vein 

 often run side by side, embedded in 

 the same connective tissue, a section 

 of this will show at a glance the 

 structure of each and the difference 

 between them. In the capillaries 

 the walls are considerably reduced, 

 and consist simply of the endothe- 

 lium, thus bringing the blood into 

 more intimate relation with the 

 tissues. 



We have already seen that 

 the blood consists of a fluid portion, 

 the plasma, in which are suspended 

 countless numbers of minute nucle- 

 ated cells, the corpuscles. The 

 plasma is rich in proteid matter, and 

 one of the compounds present in it is 

 termed fibrinogen. This substance, 

 when exposed to air, as for example 

 when a blood-vessel is cut, forms an 

 interlacing network of threads of 

 another and solid body, fibrin. The 

 fibrin mass entangles all the corpus- 

 cles, and as it shrinks squeezes out 

 a clear pale yellow- coloured fluid, 



the serum. This is the well-known phenomenon of clotting, and the 

 serum represents the plasma, from which the fibrinogen has been 

 deposited as solid fibrin threads. The corpuscles are of two kinds, 

 colourless, the leucocytes, and red corpuscles or erythrocytes. Several 

 varieties of the former are recognised, according to the number of 

 nuclei they contain. They possess the power of being able to creep 

 through the walls of a capillary without leaving a wound in it. 

 This they frequently do, and are in consequence to be found widely 

 distributed through the body and also in the lymph. They appear 

 to play the part of scavengers, and help in the removal of waste 

 matters from the tissues. In addition to this, they have the power 

 under certain conditions of ridding the body of bacteria, and hence 

 are of importance in helping to repel bacterial invasion a process 



FIG. 22. Transverse section 

 through a small artery and 

 vein. From Gray. 



A, artery ; V, vein ; e., epithelial 

 lining ; m., middle muscular and elastic 

 coat, thick in the artery, much thinner 

 in the vein ; a., outer coat of areolar 

 tissue (magnified 350 diameters). 



