HEART 



Birds and Mammals. In these Classes, the atrial and 

 ventricular septa arc always complete, and there is no longer any 



muscular walls of 

 This 



mixture of the arterial and venous blood. 



the ventricle are strongly developed and very compact. 



particularly the case in the left 



ventricle, on the inner wall of 



which the -papillary muscles are 



very strong : the left ventricle is 



partially surrounded by the right, 



the cavity of the latter having 



a semilunar transverse section, 



and its walls being much thinner 



than those of the former (Fig. 



315). 



Both in Birds and Mammals 

 the blood from the head and body 

 passes by means of the precavals 

 and postcaval into the right 



is 



FIG. 315. TRANSVERSE SECTION 

 THROUGH THE VENTRICLES OF 

 Grit* r/'ii('i-ea. 



S, 



atrium 1 as does also that from 



septum vent riculor urn ; TV, 

 right, and Vg, left ventricle. 



venosus especially in 



the walls of the heart through 

 the coronary veins, 2 and the sinus 



Mammals is scarcely recognisable (Figs. 316-318) : the right 

 atrium is separated from the right ventricle by means of a well- 

 developed valve. In Birds this valve, like that of Crocodiles, 

 is very large and muscular, while in most Mammals it consists 

 of three membranous lappets (tricuspid valve) to which are 

 attached tendinous cords, arising from the papillary muscles. 

 In Birds and Monotremes the left atrio-ventricular aperture is 

 provided with a valve consisting of three membranous folds : in 

 other Mammals there are only two folds, and the valve is therefore 

 known as the bicuspid or mitral. 3 Three semilunar pocket-like valves 



1 Considerable variations are seen in Birds in this respect ; all three veins 

 may open together, or separately ; or again, the left precaval may open inde- 

 pendently, and the right precaval and postcaval together. 



' These open into the base of the left precaval (roronary Ktnitx), the rest of 

 \vhieh disappears in certain Mammals. Coronary arteries are also present, arising 

 from the base of the aorta. 



3 There are no chorda 3 tendinere in Monotremes, the heart of which in many 

 other respects resembles that of the Sauropsida. One of the lappets on either 

 side is attached to the septum ; on the left side there is one lateral lappet, and on 

 the right two. In Marsupials, the number of lappets in the " tricuspid " valve 

 varies between two and five. As regards the papillary muscles, the following- 

 may be taken as a type in Placental Mammals. In the right ventricle the two 

 lateral lappets are connected with three papillary muscles or groups of muscles ; 

 of these the strongest is lateral, arising from the septum or from the outer wall 

 of the ventricle, the others being respectively ventral and dorsal, the latter of 

 which is the weakest of the three. The median lappet is usually connected by 

 chord* tendinea? directly to the septum, at which point small papillary muscles 

 may be present. In the left ventricle there is a ventral and a dorsal group of 

 papillary muscles, from which the chorda? teudinese exclusively arise ; the lateral 

 valve is also connected directly with the lateral wall of the ventricle by a few 

 tendinous cords. 



