CHAP. IX.] 



THE VASCULAR SYSTEM. 



105 



versely, longitudinally, obliquely, and in the apex of the ven- 

 tricles take a spiral turn or twist. The muscular walls of the 

 auricles are much thinner than those of the ventricles, and the 

 wall of the left ventricle is thicker than that of the right. 

 This difference in bulk is 

 to be accounted for, as 

 we shall see later on, by 

 the greater amount of 

 work the ventricles, as 

 compared with the auri- 

 cles, have to do. The 

 muscular walls of the 

 heart are abundantly 

 supplied with blood 

 and lymph. The nerves 

 which supply the heart 

 are partly derived from 

 the cerebro-spinal system, 

 and partly from the sym- 

 pathetic system. Con- 

 nected with the nerve 



fibres supplying the heart FlG . 79 . _ ANTERIOR VIEW OF HEART, Dis- 



are groups of nerve cells S ECTED, AFTER LONG BOILING, TO SHOW THE 



SUPERFICIAL MUSCULAR FIBRES. (Allen Thom- 



Or ganglia. son.) The aorta (6') and pulmonary artery (a') 



The heart is Covered have Deen cut short close to the semilunar valves. 



' a, right ventricle; 6, left ventricle; c, c, groove 



as mentioned above, by a between ventricles; d, d', right auricle; e, e' , left 



TTiPn-ihranrm* pnvprino* in auricle ;/, superior vena cava; g' t g", right and 



1 left pulmonary veins. The fibres are seen run- 



the form of a Sac. This nin g in a circular, oblique, transverse, and longi- 



, . tudinal direction. 



membranous sac, or peri- 

 cardium, is one of the serous membranes of the body. 1 It is a 

 sort of double bag ; one half of the bag, called the visceral por- 

 tion (viscus, organ), is closely adherent to the heart substance, 

 and also covers the great blood-vessels for about an inch and a 

 half (38 mm.) from the base of the heart ; the other half, the 

 parietal portion, is continuous with, and reflected over, the vis- 

 ceral portion, so that it loosely envelops both it and the heart. 

 The pericardium forms a completely closed sac ; its internal 

 surfaces are very smooth and polished, they are lined by endo- 

 thelium (see note on p. Ill) and secrete a small quantity of 

 1 See note on serous membranes at end of chapter. 



