84 PHYSIOLOGY AND TEMPERANCE. 



The heart is a hollow organ, and the cavity is divided into 

 two separate compartments by a muscular wall running from 

 top to bottom. Each compartment is divided into two parts, 

 thus making four chambers. The upper chambers are called 

 respectively the right and the left auricle. The lower are 

 known as the right and left ventricles. 



The walls of the heart are made up chiefly of muscular 

 tissue, in which are nerves and small blood-vessels to nourish 

 the structure. The chambers are lined with a smooth, fibrous 

 membrane, the endocardium. The heart is surrounded by a 

 closed sac, which is also conical in shape, called the pericar- 

 dium. The base of this sac is below, while at its upper part, 

 or th'e part corresponding to the base of the heart, it is 

 folded on to that organ, and becomes its outer covering; 

 hence the heart is really surrounded by a closed bag, the 

 inner layer of which is firmly attached, while the outer 

 layer is large and loose. Between the two layers is a small 

 amount of watery fluid to moisten the surfaces and prevent 

 friction from the constant movements of the heart. 



7. The Valves Of the Heart.. The heart is a double 

 organ, the two right chambers containing the dark and the 

 two left the bright red blood. The right side receives into 

 its auricle the venous blood througTr^tTm large \t:ii. 

 from above and the other from below, the r<'iia cave 1 . As 

 the auricle contracts, the blood is forced into the right 

 ventricle through an opening in the partition between the 

 two chambers. The opening is guarded by a valve, made 

 up of three triangular folds of membrane, and hence called 

 the tricuspid valve. As soon as the ventricle begins to 

 contract this valve closes, and the blood is sent onward 

 into a large vessel, the pulmonary artery, and this, dividing 

 into two, carries the blood to the lungs. To prevont any 

 return of blood when the ventricle relaxes, the pulmonary 



