30 \N V.TOM1 1\ A NUTSHELL. 



righl renal vein. These veins of the righl side enter into the inferior vena 

 cava :iinl have valves. 



The heart is a voluntary muscle (straited) in structure bill is presided over 

 by the sympathetic nerve which makes it involuntary in action. The average 

 weighl of the heart is about 11 ounces. Its covering is called pericardium, its 

 lining endocardium, and the muscle itself is called myocardium. It has four 

 chambers, the two upper ones arc the auricles (ears) and the two lowerones 

 ventricles (bellies). The two auricles contract at the same time that the ven- 

 tricle- expand, and vice versa. The contraction is called the systole and the 

 expansion the diastole. The systole is the work, and the diastole is the rest. 



Capillaries arc not found in cartilage- epithelium — epidermis. The net 

 work of capillaries is very close in the lungs mucous membrane — muscle and 

 adipose tissue and in the gray matter of the brain and cord. The capillary 

 meshes an' wider and the vessels fewer in the fascia — aponeurose — tendons 

 and ligaments. In their simplest form the capillaries consisl only of endo- 

 thelial lining of the arteries and veins with which they are connected, being 

 both elastic ami contractile, their lumen varies under pressure. They are 

 microscopic and in order to get a drop of blood many musl be ruptured. Arter- 

 ies and veins have nerves called vaso-motor which forms a net-work around 

 the muscular coat, especially of the arteries. 



\ EINS. 



Veins are found in almosl all the tissues and carry the blood from the cap- 

 illaries to the heart. A.s they are generally larger and more numerous than 

 the arteries, their total capacity greatly exceed- that of the arterial system. 

 Not only the smaller branches, but even the larger trunks communicate with 

 each other very freely. The venous system has two distind subdivisions, the 

 pulmonary ami the systemic. 



The pulmonarj veins aid in the pulmonary circulation and convey arterial 



bl 1 to the left auricle; they are but slightly larger than their arteries, and have 



no valvt s. 



The systemic veins aid in the general circulation and convey venous blood 

 to the righl auricle. They are divided (systemically) into three sets; super- 

 ficial, deep, and sinuses. The superficial or cutaneous veins lie just under the 

 skin between the layers of the superficial fascia; they pierce the deep fascia to 

 empty into the deep veins. The deep veins usually accompany the arteries 

 and are enclosed in the same -heath. There i- generally only one vein with 

 each of the larger arteries as the subclavian but two (venae comites) ac- 

 company each of the -mailer arteries as the radial, and all the arteries of the 

 lower extremities excepl the femoral artery, one lying on each side of them ; in 

 certain regions they do nol accompanythe arteries. The sinuses are venous 

 channels which are found only in the skull ami are formed by a separation of 

 the layer- of the dura mater: they are lined by endothelium. 



The arteries and veins have threecoats; fir-t the tunica intima, second the 

 tunica media and third the tunica externa (adventitia). This third coat is the 

 "tily one not ruptured in ligation of an arterv. The tunica media has muscular 



