278 THE BIOLOGY OF THE FROG chap. 



separate. Briicke's observations were extended and in most 

 points confirmed by Sabatier in 1873. The interpretation 

 of the latter author has been followed by Gaupp in his 

 recent revision of Ecker's " Anatomie des Frosches." 



When the auricles contract, the blood from the left auricle, 

 which has come in from the pulmonary vein and is therefore 

 oxygenated, is forced into the left side of the ventricle, while 

 the impure blood from the right auricle, which comes 

 through the sinus venosus, pours into the right side and 

 middle portion of the ventricle. The blood from these 

 different sources is prevented from becoming mixed by 

 being received into the slit-like chambers in the ventricular 

 wall. During the contraction of the ventricle the impure 

 blood lying near the opening of the bulbus naturally passes 

 out first, while the pure pulmonary blood from the left side is 

 forced out only toward the close of the ventricular contrac- 

 tion. When the ventricle first contracts, the wall of the bul- 

 bus cordis is relaxed, and the impure blood flows freely over 

 the edge of the spiral valve into the left compartment, 

 whence it is free to issue into the pulmo-cutaneous arches 

 through their common opening. Now the blood is under 

 less pressure in the pulmo-cutaneous arches than in the 

 others, because its route is shorter and there are no impedi- 

 ments to its flow. In the carotid arches the blood meets 

 with a partial obstruction in the carotid gland, and at the 

 outer ends of the systemic arches there is a small valve 

 (valvula paradoxica), which also tends to retard its flow. 

 The blood first issuing from the heart takes the line of least 

 resistance, namely the pulmo-cutaneous arches, and is 

 forced through the first two pairs of arches only when it has 

 no easier avenue of escape. Toward the close of the con- 

 traction of the ventricle, when the pure blood is passing out, 

 there is a contraction of the bulbus cor(!lis. This brings the 



