XIII PHYLUM CHORDATA * 575 



be double, runs forwards to open into tlie base of one of the great 

 veins of the precaval system by a valvuhir aperture. 



The general statements which have been given with regard to 

 the heart of the Rabbit (p. 4'G2) hold good for the Mammalia in 

 general. The sinus venosus is never distinct from the right 

 auricle ; of its valves, which are more completely retained in 

 tlie Edentata than in the other orders, the right gives rise to the 

 Eustachian valve — a membranous fold, often fenestrated in the 

 adult, extending from the right wall of the postcaval to the edge 

 of the foramen ovale (annulus ovalis); while the left becomes 

 merged in the auricular septum, helping to complete the aimulus 

 ovalis behind. Each auricle has an auricular appendix. The 

 right auriculo-ventricular aperture has a three-lobed (tricuspid) 

 valve, and the left a two-lobed (bicuspid or mitral), with choixhe 

 tendinea^ and musculi papillares. In all Mammals the openings 

 of the pulmonary artery and aorta are provided with three-lobed 

 semilunar valves. 



The single aortic arch, situated in all Mammals on the left side, 

 varies greatly in the way in which it gives off the main arterial 

 trunks. Sometimes a single large trunk passes forward from the 

 arch of the aorta and gives rise to both carotids and both sub- 

 clavians. Sometimes there are two main trunks — right and left 

 innominate arteries — each giving rise to the carotid and subclavian 

 of its own side. Sometimes there is a right innominate giving 

 off right carotid and right subclavian, the left carotid and left 

 subclavian coming off separately from the arch of the aorta ; or, 

 as in the Rabbit, an innominate may give origin to the right 

 subclavian and both carotids, the left subclavian alone coming off 

 separately. 



In Monotremes and Marsupials, in most Ungulates, and in 

 the Rodentia, Insectivora, and Chiroptera, both right and left 

 precavals persist ; in the others the left aborts, its vestige giving 

 rise to the coronary sinus. In the Monotremes the openings of 

 all three cavals are provided with valves, only vestiges of which 

 exist in the other groups. In the Monotremes all the pulmonary 

 veins open by a common trunk. In the Metatheria and Eutheria 

 the four veins sometimes open separately, sometimes the two veins 

 of each side unite to form a single lateral trunk. 



The following are some of the principal variations in the struc- 

 ture of the heart which occur in the different groups of Mammals. 

 In the Monotremes there is a deep fossa representing the fossa 

 ovalis in the auricular septum. The tricusjjid valve in Ornitho- 

 rhynchus consists of two membranous and two fleshy portions ; the 

 mitral valve is wholly membranous. In Echidna the tricuspid 

 valve is completely membranous, and consists of two portions — a 

 larger and a smaller. In the Marsupials the fossa ovalis and 

 annulus ovalis aie absent; in the uterine foetus of the Kangaroo 



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