GENERAL STRUCTURE OF THE BODY. 29 



out the tissues, forming plexuses and being especially 

 abundant where the blood is needed for other purposes 

 than local nutrition, as in the secreting glands. Their 

 diameter is so small that the red corpuscles have to pass 

 in single file and may even then be squeezed out of shape. 

 As they have no muscular tissue in their walls, they have 

 no power of contracting. Their walls, however, like 

 those of the smaller arteries and veins, are porous and by 

 virtue of this quality they play an important part in the 

 economy, since in them the exchange takes place be- 

 tween the tissues and the blood. 



The arteries in general carry freshly oxidized blood 

 and the veins blood from which the oxygen has been 

 largely used up and which contains waste material. 

 In the pulmonary system, however, the case is re- 

 versed, the pulmonary arteries conveying venous blood, 

 as it is called, from the heart to the lungs to be oxi- 

 dized and the veins returning the blood after it has 

 received its new supply of oxygen. 



The pumping of the blood through the arteries is as- 

 sisted by the contractions of the muscular coat, while 

 the elastic tissue, of which it contains a certain amount, 

 gives elasticity to the walls and enables them to stretch 

 and so to accommodate the larger blood supply forced 

 into them at each beat by the heart. The walls of the 

 veins have not the power of contracting and the blood 

 is pushed through more by gravity and the action of the 

 arteries than by any action of their own. 



The walls of all the vessels are nourished by tiny 

 blood-vessels in the outer^ coat, known as vasa vasorum, 

 and the nerves that regulate the action of the arteries 

 are the vasomotor nerves from the vasomotor center 

 in the medulla. Sufficient impulse goes from this cen- 

 ter to the blood-vessels all the time to keep them some- 

 what contracted, in a state of tone, that is, which is 

 increased or diminished as the blood supply is to be 

 diminished or increased. 



Lymphatic System. The lymphatic system also ex- 



