170 ANATOMY AND PHYSIOLOGY 



those points where they exist (Fig. 118), as the blood fills the 

 pockets from above. 



Blood-vessels possess nerves (the vaso-motors) which, by con- 

 trolling the muscular coats, regulate the amount of blood flowing 

 through them at a given time to the structures which they 



supply. (An organ at work needs more blood 



than an organ at rest.) 



They also possess tiny blood-vessels in their walls, 

 the vasa vasorum. 



All blood-vessels have sheaths of connective 

 tissue. In the case of the larger ones these are 

 quite strong and sometimes inclose a vein, an 

 artery, and a nerve together for protection. 



THE HEART 



The heart is a hollow muscular organ through 

 which the blood passes, placed behind the ster- 

 num and just above the central tendon of the 

 FlG Iig _ A diaphragm. Its average size is about five 



VEIN LAID OPEN TO inches long by three and one-half wide, and two 

 SHOW VALVES. , , , r ,. , 



and one-half thick. 



Note. The muscle tissue of the heart te called the myocardium. 



It is shaped like a cone, about five inches long and three and 

 one-half inches wide, with the base turned upward toward the right 

 shoulder and the apex pointing downward toward the left side. 

 It is composed of several layers of muscle fibers which are peculiar, 

 being involuntary and at the same time striped. 



The cavity of the heart is divided by a septum into right and 

 left portions, and as it lies in the body the right heart is nearly in 

 front of the left. Each side consists of two chambers, an auricle 

 (atrium) and a ventricle (ventriculum) (Fig. 120). 



The auricles receive blood and pass it into the ventricles. 

 Their walls are thin and flabby. The right auricle, or atrium, 

 presents two large openings for the entrance of veins, and one for 

 communication with the right ventricle. The veins are the 

 superior vena cava from the head and upper extremities; the inferior 



