94 PEACTICAL PHYSIOLOGY 



junction of the sinus with the right auricle. Entering the sinus from 

 below is a large vein, the vena cava inferior, into which open the 

 hepatic veins. Above there enter the two smaller superior venae 

 cavae. These are seen on gently displacing the auricles. The small 

 pulmonary veins enter the left auricle. 



The Contraction of the Heart. The venae cavae and sinus beat first, 

 then the auricles, and lastly the ventricle and bulbus arteriosus. The 

 blood is returned from all parts of the body to the sinus venosus, 

 whence it passes to the right auricle. From the pulmonary veins the 

 blood passes into the left auricle. The two auricles simultaneously 

 contract and expel the blood into the ventricle. The two blood streams 

 do not readily mix in the ventricle owing to the muscular meshwork 

 within its cavity. When the ventricle contracts the venous blood on 

 the right side is the first to enter the bulbus arteriusus. It is directed 

 by a spiral valve within the bulbus into the pulmono-cutaneous arteries. 

 The spiral valve is then driven over and closes the orifice of the 

 pulmono-cutaneous arch, and the blood (partly arterial and partly 

 venous) now passes into the systemic or carotid arch. The resistance 

 is least in the systemic arch, so most of the blood at first takes this 

 pathway. Finally, as the pressure increases in the systemic arch, the 

 pure blood from the left side of the ventricle is expelled into the 

 carotid artery. Between the auricles and ventricle there hangs the 

 auriculo-ventricular valve. The bulbus arteriosus contains two sets of 

 pocket-shaped valves in addition to the longitudinal spiral valve. 



The ventricle becomes smaller, harder, and pale in colour during 

 systole, as the blood is driven out of the muscular sponge-work of 

 which it is composed. It reddens in diastole. Count the beats per 

 minute. 



The Tissue of the Heart possesses Automatic Rhythmic Power. 

 Excise the heart, cutting widely round the sinus venosus, and place it 

 in a watch glass. Note the immediate effect and the after-effect 

 on the rhythm. The beats may at first intermit and then become 

 more frequent, but quickly settle down to about the same rate as 

 before. 



The Effect of Temperature on the Rhythm. Pour on the heart some 

 normal saline solution which has been cooled in ice. The frequency 

 becomes greatly lessened. Replace the cold with warm saline (25 C.). 

 The heart-beats become frequent as the temperature rises. If heated to 

 40-43 C. the heart stops still in diastole, but may recover if quickly 

 cooled. If not cooled the heart passes into the condition of heat rigor. 



Rhythmic Contraction the function of the Heart Muscle. Taking 

 another heart, cut away the sinus at the sino-auricular junction. After 



