CHAPTEE VIII. 



THE VASCULAR SYSTEM. 



GENERAL CONSIDERATIONS HEART ARTERIES CAPILLARIES VEINS LYMPHATICS 



THORACIC DUCTS. 



817. General Considerations. In the cat, as in man and the 

 higher animals generally, the tissues are supplied with blood, and 

 blood and lymph are removed from the tissues, by means of a series 

 of closed tubes or vessels. These tubes all communicate more or 

 less directly with one another, but excepting the lymphatic stom- 

 ta present no obvious openings into any other parts. This closed 

 Ties of tubes is known as the vascular system. 



The vascular system as a whole consists of two main divisions, 

 e blood vascular system and the lymph vascular system or 

 phatic system. 



818. The blood vascular system is that by which the blood is (A) conveyed to the 

 in general for their nourishment and returned therefrom ; (B) conveyed to the 

 lungs for its own improvement and returned therefrom (Fig, 92). Those parts which are 

 concerned in the transfer to and from the lungs constitute the pulm&nic division of the 

 blood vascular system, and the remainder constitute the general or systemic division. 

 While in process of transfer, the blood is said to perform either the pulmonic or the 

 systamic circulation. 



819. Subdivisions of the Blood Vascular System. There are four parts, continuous 

 with each other, but more or less distinctly differentiated : (A) A central receiving and 

 distributing organ, the heart (cardia) ; (B) tubes extending from the heart throughout the 

 lungs and the organs generally, the arteries ; these divide and diminish in size like the 

 branches of a tree, and gradually merge into (C) the capillaries, the most minute vessels, 

 which in turn unite and gradually merge into (D) the veins, which unite and increase in 

 size as they diminish in number and finally terminate in the heart (Fig. 92). 



The arteries are said to continually divide and decrease in size because the current 

 therein is from the larger vessels toward the smaller. The veins are said to unite and 

 increase in size because the blood current is from the smaller to the larger branches. 

 Again, the arteries are said to extend from the heart peripherad, that is, in the direction of 

 their blood current, while the veins are said to extend toward the heart or centrad, as the 

 current is toward the central organ. The veins differ from the arteries by having thinner 

 walls anJ by the presence of valves in many (Fig. 102, B, C). 



