64 HUMAN PHYSIOLOGY 



2. THE HEART 



i. The structure of the heart. The heart is a hollow 

 muscle, divided by a partition into two cavities, the left and 

 the right heart. Each cavity consists of a thin-walled auricle 

 and a thick-walled ventricle. The muscle fibres inclose the 

 cavities in different directions, some more or less obliquely, 

 some in the form of the figure 8, some circularly. The walls 

 of the left ventricle are thicker than those of the right 

 ventricle. 



At the boundary between the auricle and the ventricle are 

 found the auriculo-ventricular valves, on the right side three 

 (tricuspid) and on the left two (bicuspid) membranes hang- 

 ing down into the ventricle. On the free edges of these 

 membranes are the cordae tendineae, which are connected 

 with the wall of the ventricle by the papillary muscles. 



Between the left ventricle and the aorta and between the 

 right ventricle and the pulmonary artery are the three 

 pocket-like semi-lunar valves; the openings of the pockets 

 are toward the arteries. 



2. Properties of cardiac muscle. 



For the investigation of the physiological properties of the 

 cardiac muscle, the excised apex of the frog's heart is especially 

 adapted, as this contains no ganglionic cells. 



The heart muscle-fibre is cross-striated, but differs from the 

 striated skeletal muscle in : 



(a) Its structure. The cardiac muscle-fibres branch and anas- 

 tomose with each other. 



(b) Its functions : 



tx. A stimulation, if at all active, always calls forth a 

 maximum contraction of the cardiac muscle, while a 

 skeletal muscle does not give a maximum contraction with a 

 weak stimulation. 



ft. The cardiac muscle can be thrown into tetanus only 

 under certain abnormal conditions. If the cardiac muscle 

 is continuously stimulated, e.g. by a constant current or by 

 tetanizing induction shocks, generally no lasting contrac- 

 tion takes place (as in the skeletal) ; but the heart makes 

 rhythmical single contractions which at best fuse into an 

 incomplete tetanus (an irregular agitation and heaving of the 

 muscle). 



