214 ESSENTIALS OF PHYSIOLOGY. 



filled with fluid under pressure. The heart of the snail is so susceptible 

 to this stimulus that it will not beat at all unless its fibres are under 

 tension. 



THE MAMMALIAN HEART. 



In the mammalian heart the sinus, although present in early 

 embryonic life, does not exist as a separate structure after birth, but 

 is represented by a mass of specialised tissue, lying close to the entrance 

 of the superior vena cava into the heart, and extending a little way 

 along the sulcus terminalis of the right auricle. This tissue is known 

 as the sino-auricular node. 



It has already been mentioned that connecting the auricles and ven- 

 tricles is a band of tissue known as the bundle oftfis. This bundle 

 starts near the opening of the coronary sinus into the right auricle, its 

 point of origin being called the auriculo-ventricular node. It passes along 

 the top of the interventricular septum for a short distance, and then 

 divides into two branches, one of which runs down the right and the 

 other down the left wall of the septum immediately under the endo- 

 cardium. In some animals, e.g. the calf, it can be readily dissected out 

 in this part of its course as a thin band, paler than the rest of the 

 ventricular muscle. It soon breaks up into a number of very fine 

 branches which pass partly to the papillary muscles, and are partly 

 distributed over the rest of the wall of the heart. The extent of this 

 branching is well seen in fig. 79. Microscopically, the fibres making 

 up the terminal branches of the bundle are larger and paler than 

 ordinary cardiac muscle fibres, and only the peripheral part of the fibre 

 shows cross striation, the centre being protoplasmic in character ; the 

 fibres are called Purkinje's fibres. 



The Rhythm of the Mammalian Heart. Although the heart is pro- 

 vided with many nerve cells and receives in addition a nerve supply 

 from the central nervous system, its rhythm in the mammal, as in the 

 frog, is in all probability of myogenic origin and depends solely upon 

 the inherent rhythmic power of the muscle itself. It has been shown, for 

 example, that provided they are adequately supplied with oxygenated 

 blood, strips of mammalian ventricle will continue to beat for some 

 hours although they contain no nerve cells. Evidence to the same 

 effect is furnished by the heart of the embryo chick, which begins to 

 beat some days before any nerves are present in it. 



The impulse normally starts in the sino-auricular node, and, travel- 

 ling over the walls of the auricles, reaches the auriculo-ventricular node. 

 From this node, the impulse passes along the bundle of Mis to the 

 ventricles. The importance of the bundle of His is manifested by the 



