BY DR. BURDON-SANDERSON. 275 



seen in the latter, they exist as a rule in the former. In the 

 recent state, indeed, it is quite impossible to demonstrate their 

 presence in either case, but they can be detected after prepara- 

 tion with chloride of gold in the manner directed in Chap. IV. 

 69. Description of the Intrinsic Nervous System of 

 the Heart of the Frog. The heart of the frog is not known 

 to receive nerves from any source excepting the vagus. The 

 cardiac branches of this nerve, as they enter the heart (see 

 73), apply themselves to the superior vena cava close to its 

 origin, and then, after giving numerous branches beset with 

 ganglionic cells to the sinus venosus, the two nerves combine 

 to form a plexus at the upper part of the septum, between the 

 auricles. From this plexus two filaments descend, the smaller 

 along the anterior edge of the septum, the larger along the 

 posterior. On approaching the auriculo-ventricular orifice, 

 each of them exhibits a distinct bulging (Bidder's ganglia), 

 from which radiating streaks may be seen to spread towards 

 the ventricle. 



So long as the nerves are still outside of the heart they do 

 not contain any ganglion cells, nor give off any branches ; but 

 as they approach the plexus they become beset with cells, and 

 give off numerous filaments to the sinus venosus. The two 

 branches (anterior and posterior) have no special relation to 

 the two rami cardiaci from which they in common originate, 

 although Bidder finds that the anterior contains more fibres 

 from the right side, the posterior from the left. In their 

 course, both filaments give off branches, which ramify in the 

 septum or pass into the wall of the auricles. In order to see 

 these nerves, the heart must be exposed by opening the peri- 

 cardium. Its point must then be drawn upwards, the two 

 aortse divided, and the ligamentous shred which connects it 

 with the posterior surface of the pericardium cut through. 

 The two venae cavae must then be divided as far from the heart 

 as possible, and the heart removed. If the organ is now 

 stretched on a wax plate by means of fine pins stuck into the 

 venae cavoe, one into the vena cava inferior, and one into each 

 vena cava superior, and examined under water, the two vagi 

 (rami cardiaci) can be seen where they are in relation with 

 the vena cava superior. If now the apex is drawn to the right 

 and fixed by a fourth pin, the side of the left auricle is ex- 

 posed, and may be slit open with fine scissors, so as to bring 

 into view the septum, which must then be cleared of the outer 

 wall of the auricle by careful dissection. Fig. 236 shows the 

 appearance of the septum prepared in this wa} r . 



70. Demonstration of the Special Functions of the 

 Ganglia. 1. Stannius's Experiment The heart of a frog 

 having been exposed in the usual way, a short glass rod is 

 introduced into the oesophagus. All the other organs may 



