XIII 



PHYLUM CHORDATA 



449 



pleural sacs is a space, the mediastinum (Fig. 110-1). This is divisible 

 into four parts, the anterior, the dorsal, the middle, and the ventral. 

 In the anterior part lie the posterior part of the trachea, the neigh- 

 bouring parts of the oesophagus and of the thoracic duct of the 

 lymphatic system, the roots of the great arteries and the veins 

 of the precaval system, and the phrenic, pneumogastric, "and other 

 nerves. In the dorsal part are situated the posterior part of the 

 CBsophagus, the thoracic part of the dorsal aorta, the pneumogastric 

 nerve, the azygos vein, and the thoracic duct. The middle part is 

 the widest, and lodges the heart and roots of the aorta and pulmonary 

 artery enclosed in the pericardium, the posterior portion of the pre- 

 caval veins, the phrenic nerves, the terminal part of the azygos 

 vein, and the roots of the lungs. The ventral part contains only 

 areolar tissue with the thy- 

 mus gland. The pericardial 

 membrane enclosing the 

 heart consists of two layers, 

 a parietal, forming the wall 

 of the pericardial cavity, 

 and a visceral, immediately 

 investing the heart. Be- 

 tween the two is a narrow 

 cavity containing a little 

 fluid the pericardial fluid. 

 In general shape the heart 

 resembles the heart of the 

 Pigeon, with the apex 

 directed backwards and 

 slightly to the left, and the 

 base forwards. Like that 

 of the Pigeon, it contains 

 right and left auricles and 

 right and left ventricles, the 

 right and left sides of the heart having their cavities completely 

 separated off from one another by inter- auricular and inter- 

 ventricular partitions. 



Into the right auricle open three large veins the right and left 

 precaval veins and the single postcaval the first into the anterior 

 part, the second into the left-hand side of the posterior portion, and 

 the third into the dorsal surface (Fig. 1101). Projecting forwards 

 from it is an ear-like auricular appendix, the inner surface of which 

 is raised up into numerous cords of muscular fibres, the musculi 

 pectinati. A membranous fold, the remnant of the foetal Eustachian 

 valve, extends from the opening of the postcaval forwards towards 

 the auricular septum. The opening of the left precaval is bounded 

 behind by a crescentic fold, the valve of Thebesius. On the septum 

 is an oval area where the partition is thinner than elsewhere ; this 



FIG. 1101. Lepus cuniculus. Heart, seen from 

 the right side, the walls of the right auricle and 

 right ventricle partly- removed so as to expose the 

 cavities, ao. aorta ;/. ov. fossa ovalis ; l.pr.c. open- 

 ing of left pre-caval ; m.pap. musculi papillares ; 

 pt. c. post-caval ; pt. c'. opening of post-caval, 

 with Eustachian valve below; r.pr.c. right pre- 

 caval ; r.pul. right pulmonary artery ; sem. v. 

 semilunar valves ; tri. v. tricuspid valve. 



