76 



PEACTICAL PHYSIOLOGY 



CHAPTER XVIII. 



DISSECTION OF THE HEART. THE CARDIAC IMPULSE. 



The Sheep's Heart. The heart should, if possible, be obtained with 

 the pericardium intact, and the lungs attached to it. Open the 

 pericardium and test its strength. It is a strong, inelastic, fibrous 

 bag, and prevents the over-distension of the right heart. The parietal 

 layer of the pericardium is attached to the roots of the large vessels at 



the base of the heart, it thence 

 runs over the surface of the heart, 

 forming the visceral layer. The 

 pericardium in man is attached 

 below to the diaphragm, while 

 above it is slung to the spine by 

 the cervical fasciae. The heart is 

 thereby slung in position, and 

 cannot twist over during changes 

 of posture. 



The front of the heart is re- 

 cognised by a groove filled with 

 fat, which runs obliquely down 

 the ventricles from left to right. 

 The groove starts from about the 

 middle of the base of the ven- 

 tricles to a point a little below 

 the middle of the right margin 

 of the heart. Running up the middle of the posterior and flatter 

 surface of the heart is a similar shallow groove. The heart is 

 divided by these grooves into a right and left side, and each 

 of these is again divided by a groove containing much fat, which 

 circles round the top of the ventricles. Above this groove lie the 

 right and left auricles. Note the musculature of the left ven- 

 tricle is thick and firm, that of the right ventricle thinner. Both 

 the auricles are thin walled. The appendix of each auricle projects in 

 front at the base of the heart, as a flat, crinkled, ear-shaped bag. The 

 greater part of the auricles lies at the back and sides of the base of the 

 heart, and is concealed by the aorta and pulmonary artery. The 

 grooves on the surface of the heart mark the position of the septa, 

 which divide the heart into foui chambers. Trace the right and left 

 coronary arteries, which issue from the right and left sinuses of Valsalva 



FIG. 76. Gad's method of showing the action 

 of the cardiac valves. (Fredericq.) 



