II THE HEART AND ITS VALVES 65 



some others, very much subdivided in Eodents and other groups. 

 The degree of subdivision and the proportions of the several lobes 

 frequently offer valuable systematic characters. The gall-bladder 

 may be present or absent ; it is always a diverticulum of the 

 hepatic duct. The two are never separate, as in birds, for 

 instance. 



■ Organs of Circulation. — The heart of all mammals is a com- 

 pletely four-chambered organ. In the adult heart there is no 

 communication between tlie right and left halves. The auricles 

 are comparatively thin- walled, the ventricles thick -walled, in 

 relation to the amount of work that they have severally to per- 

 form. The right ventricle, moreover, which has only to drive 

 the blood into tlie lungs, is much thinner-walled than the left 

 ventricle, which is concerned with the entire systemic circulation. 

 The exits of the arteries and the auriculo-ventricular orifices are 

 guarded by valves, which are so arranged as only to permit the 

 blood to flow in the proper direction. But these valves have a 

 morphological as well as a physiological interest. At the origin 

 of each artery, the aorta and the pulmonary, there is a row of 

 three watch-pocket valves, as they have been generally termed on 

 account of their form. These three valves meet accurately in 

 the middle of the lumen of the arterial tube when liquid is 

 poured into them from above, and thus completely occlude the 

 orifice. The auriculo-ventricular valves differ in structure in the 

 two ventricles. That of the left ventricle has only two flaps, 

 and is therefore often spoken of as the bicuspid or mitral valve. 

 Both these flaps are membranous,' and together they completely 

 surround the exit from the auricle into the ventricle. The edges 

 of the valve are bound down to the parietes of the heart by 

 numerous branching tendinous threads, the chordae tendineae, 

 which often take their origin from pillar-like muscles arising 

 from the walls of the heart, the so-called inusculi papillares. The 

 valve of the right ventricle is composed of three flaps, and is 

 therefore often spoken of as the tricuspid valve ; it is in the same 

 way membranous, and has chordae tendineae and musculi 

 papillares connected with it. The disposition of the musculi 

 papillares and their number differ in different mammals, but no 

 exhaustive study has as yet been made of the arrangements in 

 different groups ; the amount of individual variation even is not 

 known, though it is certainly considerable in some cases, for in- 

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