TRANSECTION OF THE AURICLES. 335 



dices projecting on either side of the aorta and pulmonary artery. Of these two vessels, 

 the former is seen to be nearly in Hue with the auricular septum, while the latter, at this 

 level, is ventro-sinistrad of it. The middle of the length of the septum auriculare should 

 have been thinned to indicate the position of the fossa ovalis ( 868). In the left auricle 

 the two ends of the line which is interrupted by the abbreviation FF. point respectively 

 to the positions of the dextral and sinistral sinuses of the pulmonary veins ( 888) ; the 

 third or intermediate sinus is overhung by the auricular septum, and its position is indi- 

 cated approximately by the s of the word septum. A little of the complete wall of the 

 postcava remains, but the place of attachment of the prsecava is indicated only by an 

 unshaded area. 



895. Fig. 96 The Auricles and Auriculo-ventricular Orifices seen from the 



PIG. 96. TRANSECTION OF THE AURICLES, CEPHALIC ASPECT; xl.5. 



Cephalic Aspect. In addition to the features in common with Fig. 95, this figure illus- 

 trates the following points : 



(A) The relative positions of the three semilunar valves of the pulmonary artery, 

 namely, ventral, dextral and sinistral. 



(B) The fact that the aortic valves are upon a lower (more caudal) level than the pul- 

 monary, the former, indeed, not distinctly appearing unless the preparation is so held as 

 to admit the light directly into the vessel ( 866). 



The auriculo-ventricular valves are not especially well shown, but the form of the two 

 unshaded areas indicates, approximately, the fact that of the two Ucuspides ( 881), one is 

 lateral and the other septal, while of the three tricuspides ( 884), one is septal and the 

 others respectively dorsal and ventral (more strictly dorso-lateral and ventro lateral). 



In the right auricle, the word sinus designates the general cavity of the auricle aside 

 from the appendix ; but in the corner, and pointed at by the words sinus and septum, is 

 the location of the small sinus of the cardiac veins (% 887). 



896. Fig. 97. The Ventricular Cavities, from the Cephalic Aspect. In this 

 and Fig. 98, the most striking features are (A) the greater thickness of the lateral wall of 

 the left ventricle, in relation to its office of propelling the blood over all parts of the body, 

 and (B) the larger size of the cavity of the right. The difference of size is artificial, and 

 due to the less resistance offered by the thinner wall to the pressure of the injected alcohol. 



In the left ventricle is seen the cut end of one of the columnae carneae, still attached to 



