174 ZOOTOMY. 



{ductus Botalli) : shortly before this union the carotid art^gj^ 

 proper is given off and passes forwards to the head. 



205. The subclavian arteries {s.d), arising from the 

 right aortic arch just before its junction with the left, and 

 passing directly outwards to the fore-limbs. 



XXIII. Remove the heart, and open its various 

 cavities under water : first placing it on its ventral 

 face and opening the sinus venosus from the dorsal 

 side, and afterwards placing it on its dorsal face 

 and opening the auricle and ventricle from the 

 ventral side. Note 



206. The thin, smooth walls of the sinus venosus. 



207. The sinu-auricular aperture (Fig. 43, s.au), a 

 transverse slit placing the sinus venosus in communication 

 with the right auricle, and guarded by the two-lipped sinu- 

 auricular valve. 



208. The septum auricularum, a vertical partition 

 separating the auricles from one another. 



209. The musculi pectinati, forming a network on 

 the inner surface of the walls of the auricles. 



210. The auriculo-ventricular aperture, placing 

 both auricles in communication with the ventricle, and 

 divided into two by the free edge of the septum auricu- 

 larum : guarding the aperture is the auriculo-ventricular 

 valve, consisting of two semi-lunar flaps, a right ^nd a 

 left, placed back to back in such a way that each shuts 

 across one half of the auriculo-ventricular aperture. 



211. The thick, spongy walls and small cavity of the 

 ventricle. 



2r2. The origins of the great arteries: the pulmonary 

 artery arises to the left of the two aortfe, and its aperture is 

 separated from theirs by a muscular partition : the apertures 

 of the aortic arches are in the same vertical plane, that of 



