DISSECTION OF THE THORAX 421 



Observe the fleshy ridges and cords (trabeculae carneae) (0. 

 T. columnae carneae), the papillary muscles (Mm. papillares), 

 and tendinous cords (chordae tendineae). Here again are two 

 ostia, one venous (ostium venosum) and one arterial (ostium ar- 

 teriosum) ; note their relative positions. The ostium venosum 

 (0. T. left auriculo ventricular opening) is guarded by the bicus- 

 pid or mitral valve (valvula bicuspidalis [mitralis]). In it 

 study 



(a) Anterior cusp (cuspis anterior}. 



(b) Posterior cusp (cuspis posterior). 



FIG. 214. 



Cross-section of the ventricles. (After H. v. Meyer.) a, cavity of the left ventricle; 6, cavity of 

 the right ventricle. (From Gegenbaur, Lehrb. der Anat. des Mensch., Leipzig, 1899, 7 Aufl., Bd. ii. p. 

 217, Fig. 511.) 



The ostium arteriosum (0. T. aortic opening) is guarded by 

 the semilunar valves of the aorta (valvulae semilunares aortae), 

 to be studied later. 



Septum of the Ventricles (Septum ventriculorum). 



Study its anterior and posterior insertions and examine its 

 thickness in different parts. Note 



(a) Muscular septum of ventricles (septum musculare ventriculorum). 



(b) Membranous septum of ventricles (septum membranaceum ventricu- 



lorum). 



What is the embryological explanation? Make a note of the 

 importance of the septum ventriculorum in connection with con- 

 genital heart disease. 



Aorta and its Branches. 



Clean that part of the aorta lying within the thorax, as well 

 as the structures related to it. Study the position, shape, and 

 relations of the 



(a) Ascending aorta (aorta ascendens). 



(b) Arch of aorta (arcus aortae). 



(c) Thoracic portion of descending aorta (aorta descendens, pars tho- 



racalis). 



