318 ANATOMICAL TECHNOLOGY, 



by girdling it at about tlie middle of the leng-th of the organ and reflecting the attached 

 end over the great vessels so as to display the line of attachment. Its relations to both 

 the heart and the diaphragm will be mentioned in the descriptions of Fig. 99, 100. 



§ 829. Form. — When moderately distended, the heart is approximately aval in shape, 

 one end obtuse and the other pointed. 



As seen in Fig. 92, however, this oval is not regular, one side being more curved than 

 the other. This is due to the greater yielding of the thinner wall of the right ventricle 

 to the pressure of the injection. As a whole also, the organ is slightly flattened in what 

 will be found to be the dorso-ventral direction ; this may be due to the greater resistance 

 offered by the septum ventricalare to the pressure of the injection. 



§ 830. Normal Position. — Naturally, the longer axis of the heart is oblique with 

 respect to the meson. The smaller end points caudo-sinistro-ventrad. For purposes of 

 description, however, it will be more convenient to assume that the two ends of the organ 

 point respectively cephalad and caudad. 



The obliquity in position of the cat's heart is probably less than that of man (as re- 

 marked by Owen, A, III, 525), but it is nevertheless decided. In frozen section No. 11 of 

 the series from which Fig. 99 and 100 were made, the apex of the heart is at least 15 mm. 

 sinistrad of the meson. 



§ 831. Size. — Fig. 91 represents the natural size of the preparation from which it was 

 made, but the heart was from a large cat, and was somewhat distended with plaster by 

 injection. In round numbers, the heart of an average adult cat measures about 5 cm. 

 from the apex to the attachment of the prcecava and about 3 cm. across the greatest width 

 of the ventricular portion. We have not yet ascertained the average iceight of the organ, 

 but it is very considerable in comparison with the size and weight of the animal. 



§ 832. Designation oftJie Regions. — As shown in Fig. 92, 93, the cephalic and caudal 

 ends of the heart diflfer not only in shape, but in the thickness of the muscular walls. 

 Since the two cavities enclosed by the thinner cephalic walls are known as auricles, the 

 cephalic or basal region or end of the organ is spoken of as auricular ; in like manner, the 

 other end or region is called ventricular from the name of the two cavities enclosed by the 

 thicker walls. 



As seen in Fig. 95, 96, there is a partition between the auricles and another between 

 the ventricles. These septa divide the organ into two sides or portions like the sides of a 

 double house. Since the planes of the septa are approximately dorso-ventral, it is custom- 

 ary to speak of the two sides as right and left. The other two surfaces and regions are 

 then respectively dorsal and ventral. 



S 833. Recognition of the Regions. — The cephalic and caudal regions are readily distin- 

 guished both from the greater size of the former, from the more compressible nature of 

 the walls, and from the presence of the vessels which enter or leave the organ. In the 

 natural attitude of man, the base of the heart is uppermost, but this somewhat puzzling 

 feature does not appear when the animal is regarded as in the normal position of a verte- 

 brate (§ 35). 



The ventral aspect of the heart is comparatively regular, the two auricular appendices 

 (Fig. 95) projecting slightly at the sides of the two great arteries, aorta and A. pulmonalis. 

 The dorsal aspect, on the contrary, is quite irregular in the auricular portion on account 

 of the branching of the two arteries just named, and the entry of veins into the auricles 

 (Fig. 91). 



The right and left sides are of course determined by the determination of the ends and 

 the other two sides. In addition, the wall of the right ventricle is decidedly thinner than 

 that of the left, and that region is therefore the more compressible. 



