326 ZOOTOMY. 



bronchus for the anterior accessory lobe is given off from the trachea 

 immediately before its bifurcation. 



312. By scraping away the lung substance the bronchi may be further 

 followed : their cartilages soon lose the incomplete annular form, and 

 become irregular isolated patches : the branches given off from the 

 bronchi into the substance of the lung come off at an acute angle. 



XXXVI. In the heart cut away the outer walls of both 

 auricles, so as to expose their cavities, taking 

 care not to injure the venae cavae or pulmonary 

 veins. Note : 



313. The division of each auricle into a larger, smooth- 

 walled, posterior portion or atrium, and a small, anterior 

 portion or appendix auriculae, which projects over the 

 base of the ventricle, and the walls of which are strength- 

 ened internally by a network of muscular bands, the 

 musculi pectinati. 



314. The septum auricularum, or muscular division 

 wall between the auricles. 



315. The fossa ovalis, an oval area on the septum 

 rather thinner than the rest of it ; it is seen best from the 

 left side by holding the septum up to the light, and is sur- 

 rounded by a slightly raised margin, the annulus ovalis. 

 It marks the position of the foramen ovale of the foetus. 



316. The auriculo-ventricular apertures, by which 

 the auricles respectively communicate with the ventricles. 



317. The apertures of the pre- and postcaval 

 veins into right auricle : that of the right precaval is in the 

 anterior (upper) end of the auricle ; that of the postcaval 

 in the postero-dorsal region, a membranous fold, the 

 remains of the foetal Eustachian valve, extending from 

 its posterior margin towards the septum ; that of the 

 left precaval is immediately to the left of, and posterior 

 to, the Eustachian valve, and is bounded behind (below in 



