424 ZOOLOGY FOR MEDICAL STUDENTS chap. 



with the conus arteriosus. It will be recalled how in the headward 

 portion of the conus of the lung-fish (p. 384) the cavity becomes divided 

 into a dorsal pulmonary and a ventral systemic portion by the meeting 

 and fusing of the right and left longitudinal ridges. In the reptile a 

 similar subdivision of the cavity of the conus takes place in the embryo, 

 but it affects the whole length of the conus and not merely its front end. 

 Further the systemic cavity becomes also subdivided longitudinally into 

 a right and a left cavity. The conus thus contains three cavities and, 

 owing to the conus becoming shortened and having its flexure straightened 

 out as in the Amphibia, these cavities twist round one another in a right- 

 handed spiral — the pulmonary cavity starting at its cardiac end by being 

 on the right side, and ending up at its headward end by being dorsal. 

 In some reptiles, e.g. Chelonians, a further step is taken, the longitudinal 

 septa becoming split so that the conus is now resolved into three distinct 

 vessels, twisted round one another in a right-handed spiral and forming 

 the roots of the main arteries. 



During these changes in structure the conus also undergoes physio- 

 logical change for it ceases to contract rhythmically, its striped muscle 

 becoming replaced by smooth. The conus has in fact ceased to exist 

 as a constituent chamber of the heart and has become incorporated 

 into the arterial system. 



The atrium is completely divided into right and left auricle in the 

 reptiles, but the ventricle shows only the beginnings of a septum, except in 

 the Crocodiles, where it is complete. Of the three cavities, or three vessels, 

 which represent the conus two originate from the ventricle on the right 

 side of the septum, namely the pulmonary and the left systemic, while 

 on the other hand the right systemic arises on the left of the septum. 



The arterial system is laid down in the embryo on the same general 

 plan as in fishes but as development goes on there takes place a process 

 of gradual modification by which the original arrangement becomes 

 completely transformed. The general characters of this modification as 

 it takes place in a typical reptile is illustrated in Fig. 183, in which the 

 vessels shaded in the drawing are those present in the adult, while those 

 shown merely in outline are portions of the embryonic arterial system 

 that disappear in the course of development. 



In the first place it has to be noted that the longitudinal subdivision 

 of the cavity of the conus extends forwards into the hinder part of the 

 ventral aorta, this also becoming resolved into three cavities or three 

 separate vessels — pulmonary, left systemic and right systemic — con- 

 tinuous with those of the conus {p, l.S, S). 



Of the aortic arches the ventral portions of VI persist as the right 



