Il THE HEART AND ITS VALVES 65 
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some others, very much subdivided in Rodents 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 the 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 the 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 pRysiological 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. 
3oth 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 musculi 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- 
VOL. X F 
