CIRCULATION OF THE BLOOD. 



Fig. 6. 



19. This organ is enveloped in a double, membranous sac, 

 called pericardium, and is suspended in the pericardium by the 

 vessels which arise from its superior and enlarged entremity ; but 

 it does not adhere at any other point of its surface to the neigh- 

 bouring parts. 



20. The substance of the heart is almost entirely fleshy ; it is 

 a hollow muscle, the cavity of which communicates with the arte- 

 ries and veins. 



21. In man and all the mammalia, 

 as well as birds, it has four distinct 

 cavities. A thick, vertical partition 

 divides it into two halves, each one 

 forming two cavities, one above the 

 other ; a ventricle, and an auricle. 

 (Fig. 6.) 



22. The two ventricles occupy 

 the inferior part of the heart, and do 

 not communicate with each other, 

 but each one opens into the auricle 

 above it. 



23. The cavities of the left side of the heart contain arterial 

 blood, and those of the right side, venous blood. 



24. The vessels which convey arterial blood .into all the 

 organs take their origin from the left ventricle of the heart, 



Explanation of Fig. 6. The heart opened to show the cavities in the in- 

 terior of this organ. or/, right auricle of the heart, w/, right ventricle of 

 the heart, oo-, left auricle of the he rt, v>r, lei ventricle of the heart, 

 vcs. and vr,i, superior arid inf-ri r venae cavae, emptying 'into the right 

 auricle of the heart, ap, pulmonary artery going from the right ventricle to 

 the lungs, /j, pulmonary veins passing from the lungs to the left auricle 

 of the heart, a, the aorta, o, left auriculo- ventricular opening, o', 

 right auriculo.ventricular opening. The arrows point out the direction of 

 the flow of the blood in the different vessels. 



19. Has the heart any covering proper to it? How is it suspended in the 

 pericardium I Does the heart adhere to thd surrounding parts ? 



20. What is the substance of the heart ? Is the heart solid ? 



21. How many cavities has the heart? How is it divided ? What are the 

 cavities of the heart callttd ? 



22. What part of the heart is occupied hy the ventricles? Do the veil 

 tricles communicate with each other ? Dn they communicate with the auri- 

 cles ? How is the auricle situated in regard to the ventricle ? 



23. What kind of blood is cwitnined in the cavities of the left side of the 

 heart? In which side of the heart is found the venous blood ? 



24. From what part of the heart do these vessels arise which carry arte 

 rial blood ? What is the name of the great arterial trunk as it arises frorr 

 the heart? 



