1898.] SYSTEM OF THE chiropteBa. 69 



slender chordse tendiuese are attached to its free edge ; they all ai'ise 

 directly from the septum, without the intervention of musculi 

 papillares, and they are quite separate from the set belonging to 

 the outer segment, each set supplying its own part of the valve only 

 (fig. 6, p. 68). 



The pulmonary orifice is guarded by a valve of three semilunar 

 flaps, two placed venti'ally and one dorsally, as in man. 



The atrium of the Left Auricle appears on the dorsal surface o£ 

 the heai't between the systemic veins entering the right auricle and 

 the pulmonary artery. (Fig. 4, p. 66.) The tip of the appendix 

 appears on the ventral surface, the base being concealed in front 

 by the right pulmonary artery and on the left by the left preeaval 

 vein. On opening the cavity, the walls are seen to be thicker than 

 those of the right auricle, and smooth internally, a few feeble 

 musculi pectinati being found only in the appendix, which is longer 

 and narrower than on the right side and much less capacious. On 

 the interauricular septum a faint depression indicates the position 

 of the foramen ovale ; here the wall is very thin, but no communi- 

 cation exists between the auricles. 



The left pulmonary veins open by one common orifice into the 

 dorsal surface of the atrium, the right by three separate openings. 



The Left Ventricle contrasts markedly with the corresponding 

 cavity on the right side, being constructed on a much stronger and 

 more muscular plan. The outline in transverse section is circular. 

 Two large and strong papillary muscles, extending down to the 

 apex, and attached along their whole length to the outer portion 

 of the ventricular wall, send chordae tendinesB to the two seg- 

 ments of the mitral valve, each papillary muscle supplying part of 

 both segments. Fine columnse carnese, consisting of low closely- 

 set ridges, cover the interventricular septum and the wall of the 

 ventricle between the muscular attachments. 



The auriculo-ventricular aperture is oval in shape, and consider- 

 ably smaller than on the right side. The mitral valve consists of 

 two segments — the lesser is placed against the outer wall of the 

 ventricle, the greater hangs between the aortic and auriculo- 

 ventricular orifices. The aortic valve consists o£ three segments, 

 one ventral and two dorsal, and the coronary arteries arise from 

 the ventral and left dorsal sinuses of Valsalva. The structure of 

 both this and the pulmonary valve closely resembles the arrange- 

 ment in man. 



The weight ^ of the heart in P. medius is about 26 grains, approxi- 

 mately y^-g- of the body weight, compared with yot;- in man. The 

 thickness of the wall of the right ventricle at its base is 1 mm., 

 of the corresponding part of the left ventricular wall 4 mm. 

 This ratio is even greater in the smaller Bats, in Vesperugo noctula, 

 for instance, the figures are "5 mm. and 2-4 mm. 



The Aorta and its Branches. 



Arising from the left ventricle, the ascending aorta is at first 

 concealed from view in the undissected heart. The right auricular 



^ Taken from spirit-specimens. 



