THE VASCULAR SYSTEM 



387 



The Heart in Man. The human heart is a hollow muscular organ 

 about the size of the closed fist, weighing from nine to eleven ounces. 

 It is shaped like a truncated cone with its apex pointing downwards to 

 the left. It is placed asymmetrically behind the sternum, with its apex 

 just above the left fifth rib. A muscular partition extending from apex 

 to base divides it into right and left cavities, each of which is subdivided 

 into an anterior atrium or auricle and a posterior ventricle. Externally 

 the division of the heart into atria and ventricles is indicated by a groove, 

 the sulcus coronarius, and the two are separated internally by atrio- 

 ventricular valves. Two large veins, the precava and postcava, enter 

 the right atrium, while four pulmonary veins return blood from the lungs 



Afferent lymphatic vessels. 



Peripheral 



sinus., 



Lymph sinus. 



Capsule, 



Fig. 



Efierent lymphatic vessels. 

 324. — Diagram representing two stages in the development of a lymph gland. 



The left half of the diagram is an earlier stage than the right half. 

 "Text Book of Histology.") 



(From Bremer's 



to the left atrium. The pulmonary artery connects with the right, the 

 aorta with the left ventricle. 



The walls of the ventricles are thicker than those of the atria, and 

 the wall of the left ventricle is thicker than that of the right, for the right 

 ventricle pumps blood only to the lungs, while the left ventricle forces 

 blood to all other parts of the body and is correspondingly more muscular. 



The heart-wall has three layers, endocardium, myocardium, and 

 epicardium. The endocardium consists of a thin layer of connective 

 tissue and an endothelial layer continuous with that of the blood vessels. 

 The myocardium is the thick muscular layer. The fibers of cardiac 

 muscle are striped, and are peculiar in having anastomosing connexions 

 with one another. The epicardium is a thin layer of connective tissue 

 covered with the serous membrane which lines the pericardial cavity. 



