580 



COMPAEATIVE ANATOJMY. 



Pig. 327. Aortic arches of 

 Lep.idosiren paradox a. 

 a Trunk of the branchial 

 arteries. 12 3 Arterial arches; 

 the first two are united to form 

 the aorta, p Pulmonary artery. 

 b Ductus Botalli. hr Branchial 

 clefts. h/ Accessory gill. 

 ao Aorta. c Coeliac artery, 

 oe CEsophagus (after Hyrtl). 



example of this, for it is in them that the cavities of the heart 

 are first divided. In Lepidosiren a network of muscular bands is 

 continued from the wall of the auricle right through it, and forms a 



kind of partition. The auricle is thereby 

 divided into a right and left portion, but 

 there are many points by which these 

 two parts communicate with one another, 

 and they also open by a common opening 

 into the ventricle. The venous sinus now 

 opens into the right side of the auricle, and 

 a pulmonary vein passes into the left side. 

 The ventricle, also, is partly divided by 

 muscular processes ; and, in correspon- 

 dence with this, the lumen of the arterial 

 bulb is divided into two cavities, each 

 of which gives rise to special arteries. 

 These form three vessels which extend 

 along the anterior branchial arches ; the 

 most anterior on either side is connected 

 with the second one, and unites with its 

 fellow of the opposite side to form an 

 aorta {ao). These two vessels have no 

 relation to the gills. The third arch 

 gives rise to branchial arches, but it is 

 also connected by a narrow duct {b) with 

 the root of the aorta, and it is continued on as the pulmonary 

 artery {p). This arch forms, therefore, a branchio -pulmonary 

 artery, and the two anterior arches, as they do not give ofE any 

 branchial vessels, represent aortic arches. 



The circulatory apparatus of the Amphibia presents the same 

 conditions. In most, the auricle is completely divided, but in some, 

 as in Proteus, it is not so ; in Salamandra, also, there is a hole in the 

 septum. The systemic veins open into the right, the pulmonary 

 into the left auricle. The ventricle is single, and there are indica- 

 tions merely of a partition. There are membranous pouched valves 

 at the ostium atrio- ventricular e, which have the same character 

 as in Fishes. A muscular arterial bulb (Fig. 328, ba) is given off 

 from the ventricle, and its separation into two parts, which we saw 

 commencing in Lepidosiren, is completely effected. At first it gives 

 off as many as five pairs of arterial arches, but these are reduced to 

 three or four. As in Fishes, a network of branchial vessels is de- 

 veloped leading from the heart. Owing to this the arterial arches 

 are divided into efferent and afferent vessels; a branchial artery, 

 and a branchial vein, between which is the capillary network. The 

 branchial veins form the roots of the aorta. 



Each branchial artery is, however, connected with its own 

 branchial vein by a ductus arteriosus, which is a portion of the 

 primitive aortic arch. When the lungs are developed, the last 

 branchial artery either gives off a twig to the pulmonary artery, 



