78 MOTOR CENTRES OF THE HEART. 



single, at others double, and it can be seen as a white " crescent " when the heart is lifted up 

 and looked at from behind (fig. 54). The cardiac nerves proceed downwards on the auricular 

 septum, exchanging fibres in their course to join two ganglia at the auriculo-ventricular groove, 

 and known as Bidder's ganglia (fig. 57). It has been stated by one observer that the bulbus 

 arteriosus contains ganglionic cells, but this is denied by others.] 



According to Openchowsky, every part of the heart (frog, triton, tortoise) contains nerve- 

 fibres which are connected with every muscular fibre. In the auricles, at the end of the non- 



Fig. 57. Fig. 58. 



Scheme of nerves of frog's heart. Stannius's experiment. A, auricle; 



R. Remak's, and B, Bidder's gan- V, ventricle ; S. V., sinus venosus. 



glia; S.V., sinus venosus; A, The zig-zag lines indicate which 



auricles; V, ventricle; 13. A., parts continue to beat; in 2 the 



bulbus arteriosus ; vag, vagi. ventricle beats at a different rate. 



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 endocardium 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 pericardium contains (sensory) nerve-fibres. 

 The following kinds of nerve-cells are found unipolar cells, the single processes of which after- 

 wards divide ; bipolar pyriform cells (fig. 56), which in the frog possess a straight (n) and 

 usually also a spiral process (o). 



58. THE AUTOMATIC MOTOR CENTRES OF THE HEART. (1) It is 



generally assumed 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 move in rhythmical sequence from a principal central point, an impulse 

 being conducted from this centre through the conducting paths. 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 

 electrical currents), all the centres are thrown into action, and a spasm-like action 

 of the heart occurs. The dominating centre lies in the auricles, hence the regular 

 progressive movement usually starts from them. If the excitability is diminished, 

 as by touching the septum with opium, other centres seem to undertake this 

 function, in which case the movement may extend from the ventricles to the 

 auricles. According to Kronecker and Schmey, in the dog's heart there is a spot 

 above the lower limit of the upper third of the ventricular septum, which, when it 

 is injured, e.g., by destroying it with a stout needle, brings the heart to a stand- 

 still ; this has been called a co-ordinating centre. 



(3) All stimuli of moderate strength applied directly to the heart cause at first 

 an increase of the rhythmical heart-beats ; stronger stimuli cause a diminution, and 

 it may be paralysis, which is often preceded by a convulsive movement. Increased 

 activity exhausts the energy of the heart sooner. 



(4) Single very weak stimuli, which have no effect on the heart when applied 

 singly, if repeated sufficiently often, may become active owing to " summation of 

 the stimuli " (v. Basch). 



(5) Even the weakest stimulus which can excite a contraction always causes an. 

 energetic contraction, i.e., " the minimal stimulus causes a maximal effect "' 

 {Bowditch, Kronecker awl Stirling). 



