384 ZOOLOGY FOR MEDICAL STUDENTS chap. 



advance has been made from the condition seen in Ceratodus. In the 

 hinder part of the conus the same longitudinal ridge (Fig. 166, sp) is 

 seen projecting dorsalwards from the floor of the conus and dividing its 

 cavity incompletely into a right and a left half — continuous when the 

 ventricle contracts with the corresponding halves of the ventricular 

 cavity^ owing to the hinder edge of the ridge and the front edge of the 

 ventricular septum becoming approximated together. In the headward 

 portion of the conus a second ridge makes its appearance running along 

 the opposite side of the cavity. It faces the first ridge and the edges 

 of the two are closely approximated and overlap slightly so that the 

 cavity of this portion is practically completely divided into two. As 

 the conus passes forwards into the very short ventral aorta actual fusion 

 takes place so that the two ridges now form a perfectly continuous 

 septum. The two cavities separated by this septum lie not right and 

 left as they did at the cardiac end of the conus but dorsal and ventral, 

 owing to the conus in the middle part of its length having become folded 

 on itself in a peculiar manner. The cavity which is ventral at the front 

 end of the conus is that which is on the left hand at the cardiac end, 

 i.e. it is a continuation of the cavities of the left ventricle and left 

 auricle, the cavities which are filled with oxygenated blood returned 

 from the lung : it may be termed the systemic cavity of the conus. The 

 other cavity, lying dorsal to it and continuous with the cavities of the 

 heart containing venous blood, is the pulmonary cavity of the conus. 



The horizontal floor separating systemic and pulmonary cavities is 

 continued into the ventral aorta and gradually slopes dorsalwards at its 

 headward end, so as to merge into the roof of the ventral aorta. The 

 pulmonary cavity of the ventral aorta thus becomes obliterated at its 

 front end by its floor becoming coincident with its roof. 



The ventral aorta is greatly shortened, and the four aortic arches 

 (III- VI) spring from it close together on each side. The horizontal floor 

 which as already mentioned divides the cavity into a systemic and 

 pulmonary portion passes into the roof at a level just between arches 



IV and V, so that arches III and IV arise from the ventral (systemic) 

 cavity, arches V and VI from the dorsal (pulmonary) cavity. Arches 



V and VI join together before opening into the aortic root dorsally and 

 the pulmonary artery branches off from the short common portion so 

 formed. 



It follows from the arrangements which have been described that the 

 oxygenated blood reaching the heart by the pulmonary vein and the 

 de-oxygenated blood reaching the heart by the sinus venosus have before 

 them two distinct routes. The former is poured into the left auricle and 



