THE VASCULAR SYSTEM 171 



SO that the blood from the vein may return partly by the latter 

 vessel. 



The pulmonary veins convey the pure blood from the lungs 

 to the left atrium. There are three main trunks from each 

 lung. These six veins are arranged in pairs (Fig. 77), constitut- 

 ing a left pair from the left lung; a median pair, composed 

 of one branch from the right and one from the left lung; and 

 a right pair from the right lung. Each pair forms a common 

 trunk before entering the atrium. In order to demonstrate 

 these veins one must remove the heart and lungs from an in- 

 jected cat, and carefully dissect away the fatty and connective 

 tissues enveloping the vessels. 



The peripheral connection between the arteries and veins 

 is by means of capillaries, which are microscopic vessels with 

 an extremely thin wall composed of a single layer of epithelial 

 cells. These capillaries are so abundant everywhere in the 

 flesh that a needle cannot be inserted without penetrating 

 some. They form a kind of mesh or network, so that every 

 cell may be supplied w^ith oxygen and food and discharge its 

 waste matter. All the blood carried to any portion of the 

 body by the arteries is not returned by the veins, as a consider- 

 able amount of the plasma and some of the white corpuscles 

 escape through the thin capillary walls and are returned to the 

 circulation by a system of vessels called lymphatics (Fig. 91). 



DEVELOPMENT OF THE VASCULAR SYSTEM 



In the cat as in all other vertebrates the vascular system 

 has its origin in the mesenchyme. Small spaces or clefts appear 

 among the mesenchymal cells, enlarge and fuse together to form 

 a network or plexus of channels. Following certain laws of 

 hydrodynamics the blood stream comes to flow along the most 

 direct line of least resistance. The channels thus formed are 

 usually oriented with respect to the longitudinal axis of the 

 body. Those carrying blood toward the central organ of 

 circulation develop into the venous system, while those carrying 

 the blood stream peripherally form the arterial system. 



