486 SANGUIFEROUS SYSTEM. 



left superior or systemic portion, which communicate by a 

 wide opening resulting from the deficiency of the septum 

 near the base of the ventricle. The parietes of the systemic 

 portion of the ventricle are thick and strong compared with 

 the small interior cavity, so as to propel the systemic blood 

 through the whole extent of their elongated trunk, and the 

 numerous free fleshy columns which traverse its interior, 

 tend to mingle the venous with the arterialized blood poured 

 into its cavity. The septum of the auricles is thin and dia- 

 phanous, and sends from each side of its inner margin, at 

 the base of the ventricle, a crescentic membranous fold, 

 which forms a distinct semilunar valve over each auriculo- 

 ventricular orifice, to prevent the return of the blood during 

 the contraction of the ventricle. The approximated open- 

 ings of the two systemic aortse, in the strong upper left 

 compartment of the ventricle, are provided with two semi- 

 lunar valves, and two similar folds are observed at the 

 opening of the pulmonary artery in the right inferior portion 

 of the ventricle. The right or pulmonic portion of the 

 ventricle is more capacious than the left, and its parietes 

 are less thick, muscular, and cribriform. 



The common trunk of the right and left aortee, on leaving the 

 systemic portion of the ventricle, soon divides to embrace the 

 trachea and oesophagus- in the aortic arch, as in other cold- 

 blooded vertebrata ; and the two trunks again anastomose a 

 little posteriorly, under the vertebral column, to form the pos- 

 terior aorta, which extends backwards along the median line of 

 the body to the end of the tail. The right arch of the aorta, 

 in its course upwards and backwards, gives off a consi- 

 derable thyroid branch, and a large cephalic azygous artery 

 from which the common carotids of both sides originate. 

 The common carotids divide near the head, into a larger 

 external, and a smaller internal carotid artery, and the lat- 

 ter of these branches supplies the place of the vertebrals 

 within the cranium. The right aortic arch also sends off, 

 near its junction with the left, a single median subvertebral 

 artery which extends forwards, and supplies the vertebral or 

 spinal and intercostal arteries to each side of the anterior part 

 of the body. The common cephalic artery in advancing to 

 the head, sends numerous twigs to the resophagus and the 

 trachea, before it divides to form the common carotids. The 



