THE VASCULAR SYSTEM. 639 



obliquely backwards and towards the left, and divides it in two chambers ; 

 the right one of which remains continuous with the sinus venosus, while 

 the left one is completely shut off from the sinus, though it soon enters 

 into communication with the newly established pulmonary veins. The 

 truncus arteriosus 1 is divided into a posterior conus arteriosus (pylangium) 

 and an anterior bulbus (synangium). The former is provided with a 

 proximal row of valves at its ventricular end, and a distal row at its anterior 

 end near the bulbus. It is also provided with a longitudinal septum, which 

 is no doubt homologous with the septum in the conus arteriosus of the 

 Dipnoi. The bulbus is well developed in many Urodela, but hardly exists 

 in the Anura. 



In the Amniota further changes take place in the heart, 

 resulting in the abortion of the distal rows of valves of the conus 

 arteriosus 2 , and in the splitting up of the whole truncus arteriosus 

 into three vessels in Reptilia, and two in Birds and Mammals, 

 each opening into the ventricular section of the heart, and 

 provided with a special set of valves at its commencement. In 

 Birds and Mammals the ventricle becomes moreover completely 

 divided into two chambers, each communicating with one of the 

 divisions of the primitive truncus, known in the higher types 

 as the systemic and pulmonary aortae. The character of the 

 development of the heart in the Amniota will be best understood 

 from a description of what takes place in the Chick. 



In Birds the originally straight heart (fig. 109) soon becomes doubled up 

 upon itself. The ventricular portion becomes placed on the ventral and 

 right side, while the auricular section is dorsal and to the left. The two 

 parts are separated from each other by a slight constriction known as the 

 canalis auricularis. Anteriorly the ventricular cavity is continued into the 

 truncus, and the venous or auricular portion of the heart is similarly con- 

 nected behind with the sinus venosus. The auricular appendages grow out 

 from the auricle at a very early period. The general appearance of the 

 heart, as seen from the ventral side on the fourth day, is shewn in fig. 360. 

 Although the external divisions of the heart are well marked even before 

 this stage, it is not till the end of the third day that the internal partitions 

 become apparent ; and, contrary to what might have been anticipated from 

 the evolution of these parts in the lower types, the ventricular septum is the 

 first to be established. 



1 For a good description of the adult heart vide Huxley, Article "Amphibia," in 

 the Encyclopedia Britannic a. 



2 It is just possible that the reverse may be true, vide note on p. 640. If however, 

 as is most probable, the statement in the text is correct, the valves at the mouth of 

 the ventricle in Teleostei are not homologous with those of the Amniota ; the former 

 being the distal rov/ of the valves of the conus, the latter the proximal. 



