64 ZOOTOMY. 



141. The thyroid gland, a flattened rounded body, of 

 a deep red colour, situated immediately in front of the 

 anterior end of the ventral aorta. 



XXXIV. Cut through the ventral aorta and posterior innominate 

 arteries close to the bulbus arteriosus, and turn the heart 

 backwards, so as to expose the pericardial cavity. Note 



142. An aperture in the centre of the posterior dorsal region of the 

 pericardium : by passing a probe into this it will be found to lead into 

 a funnel-shaped cavity which soon divides into two membranous canals. 

 These the pericardio-peritoneal canals pass backwards along the 

 ventral wall of the gullet, and open each by a widish aperture, thus 

 placing the pericardium in communication with the peritoneal cavity. 



XXXV. Remove the heart entirely, by cutting through 

 both ends of the sinus venosus and the membrane 

 by which it is united to the pericardium : make 

 . out carefully the relations of the various parts of 

 the heart ( 82), then cut open, first the auricle and 

 sinus venosus from the dorsal side, and afterwards 

 the ventricle and conus arteriosus from the ventral 

 side. Make out 



143. The thin, smooth walls of the sinus venosus. 



144. The thin walls of the auricle, strengthened by a 

 complicated network of muscular fibres, the musculi 

 pectinati. 



145. The large sinu auricular aperture guarded by 

 the two membranous flaps of the sinu-auricular valve. 



146. The circular auriculo-ventricular aperture, 

 guarded by the two long flaps of the auriculo-ventricular 

 valve, which are attached round the margins of the aper- 

 ture, and hang down into the ventricle. 



147. The small, horseshoe-shaped cavity of the ventricle, 

 and its immensely thick walls, strengthened internally by 

 muscular ridges or columnae carneae. 



148. The three longitudinal rows of aortic valves in 



