98 



MOTOR CENTRES OF THE HEART. 



ganglia lie near tho orifice of the superior vena cava in birds the 

 largest ganglion (containing thousands of ganglionic cells) lies pos- 

 teriorly where the longitudinal and transverse sulci cross each other. 

 Fine branches, also provided with small ganglia, proceed from these 

 ganglia, and penetrate the muscular walls of the auricles and ventricles. 

 Nerves of the Frog's Heart. In the frog there is a large ganglion 

 (EemaJc's) near the fibres of the vagus within the wall of the sinus 

 venosus. Branches of the vagus proceed from this ganglion along the an- 

 terior and posterior walls of the auricular septum, and each of these con- 

 tains a ganglion in the auriculo-ventricular groove, these aggregations 

 of ganglion cells constituting Bidder's ganglion. Fine branches proceed 

 from this ganglion, but they can be traced only for a short distance, so 

 that the greater part of the ventricle appears to be devoid of nerves. 



According to Opeuchowsky, every part of the heart (frog, triton, tortoise) con- 

 tains nerve-fibres which are connected with every muscular fibre. In the auricles, 

 at the end of the non-medullated fibre, a tri-radiate nucleus exists which gives off 

 fibrils to the muscular bundles. 



There is a network of fine nerve-fibres distributed immediately under the endo- 

 cardium these fibres act partly in a centripetal direction on the cardiac ganglia, 

 and are partly motor for the endocardial muscles. The parietal layer of the peri- 

 cardium contains (sensory) nerve-fibres. The following kinds of nerve-cells are found 

 unipolar cells, the single processes of which afterwards divide ; bipolar cells (Fig. 

 31a), which in the frog possess a straight (n) and usually also a spiral process (o). 



58. The Automatic Motor Centres of the Heart. 



(1.) We must assume that the nervous centres which excite the 

 cardiac movements, and maintain the rhythm of these movements, lie 



within the heart, and that they are probably 

 represented by the ganglia. 



(2.) There are not one, but several, of these 

 centres in the heart, which are connected with 

 each other by conducting paths. As long as 

 the heart is intact, all its parts are made to 

 move in rhythmical sequence from a principal 

 central point, an impulse being conducted 

 from this centre through the conducting paths 

 (Bonders). What the "discharging forces" 

 of these regular progressive movements are, is 

 unknown. If, however, the heart be subjected 

 to the action of diffuse stimuli (e.g., strong 

 Fig. 31a. electrical currents), all the. centres are thrown 



Pyriforna ganglionic bi- into ti d spasm _iik e action of the heart 

 polar nerve-cell from 

 the heart of a frog occurs. The dominating centre lies 'in the 



ifstrafght processf S^ mri des, hence the regular progressive move- 

 spiral process. ment usually starts from them. If the excit- 



