CHAP. VIII.] THE VASCULAK SYSTEM. 



83 



one another so as to form a kind of network or sponge-like sub- 

 stance. The arrangement of the fibres differs in the auricles 

 and the ventricles, and is very intricate ; the fibres run trans- 

 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 FIG. 64. ANTERIOR VIEW OF HEART, DIS- 

 whioh mirmlv thp hpart SECTED > AFTER LONG BOILING, TO SHOW THE 

 >uppiy G SUPERFICIAL MUSCULAR FIBRES. (Allen Thorn- 



are partly derived from son.) The aorta (&') and pulmonary artery (a') 

 ,1 i i have been cut short close to the semilunar valves. 



the cerebro-spmal system, a> right ventricle . 6> left ventric ie: c,c, groove 



and partly from the Sym- between ventricles ; d, d', right auricle ; e, e', left 



,| ,. p auricle; /, superior vena cava; g', g", right and 



patnetl System. l^On- ]eft p U i monar y veins. The fibres are seen run- 



nected with the lierve niu S in a circular, oblique, transverse, and longi- 



^., , . , tudinal direction. 



fibres supplying the heart 



are groups of nerve cells, or ganglia. 



The heart is covered, as mentioned above, by a membranous 

 covering in the form of a sac. This 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 from the base of the heart ; the other half, the parietal 

 portion, is continuous with, and reflected over, the visceral por- 

 tion, so that it loosely envelops both it and the heart. 

 1 See note on serous membranes at end of chapter. 



