Chap. XI] THE VASCULAR SYSTEM 185 



Division. — As they proceed in their course they divide into 

 branches, the division taking place in different ways. 



(1) An artery may at once resolve itself into two branches of 

 nearly equal size (dichotomous, division, or splitting in two). 



(2) It may give off several branches in succession and still 

 maintain its character as a trunk. 



(3) It may give off one branch that divides into three equal 

 branches. In this case the parent branch is called an axis. 

 Example — coeliac axis. 



Anastomosis or Inosculation. — The distal ends of arteries unite 

 at frequent intervals, when they are said to anastomose, or inoscu- 

 late. Such inosculations admit of free communication between 

 the currents of the blood, tend to promote equality of distribution 

 and of pressure, and to obviate the effects of local interruption. 

 It is this arrangement which allows of the ligating of veins and 

 arteries during operations, or after injuries, without serious inter- 

 ference with the circulation in the part. 



Plexus. — When a number of arteries form many inosculations 

 within a limited area, it is described as a plexus or network. 

 Arteries commonly pursue a tolerably straight course, but in 

 some parts of the body they are tortuous. The facial artery, in its 

 course over the face, and the arteries of the lips are extremely 

 tortuous, so that they may accommodate themselves to the 

 movements of the parts. 



Divisions of the vascular system. — The blood-vessels of the 

 body are arranged in two main systems, namely, the pulmonary 

 and the general or systemic. 



The pulmonary system. — This is the lesser system and pro- 

 vides for the circulation of blood from the right ventricle to the 

 lungs, and then back to the left auricle. This is called the pul- 

 monary circulation. 



Blood-vessels of the pulmonary system. — The blood-vessels 

 of the pulmonary system are (1) the pulmonary artery and all its 

 branches, (2) the capillaries which connect these branches with the 

 veins, and (3) the pulmonary veins. 



The pulmonary artery. — The pulmonary artery conveys the 

 dark blood from the right side of the heart to the lungs. The main 

 trunk is a short, wide vessel (diameter about one inch, or 30 mm.) 

 which arises from the right ventricle and runs for a distance of 



