70 DE. X. H. ALCOCK OlS THE VASCULAR [Feb. 1, 



appendix and the infundibulum of the right ventricle co^■er it on 

 the ventral aspect. The right auricle and the right precaval vein 

 conceal it on one side, the pulmonary artery on the other, and on 

 the dorsal surface the right pulmonary artery crosses above it. A 

 comparatively short distance remains therefore between the surface 

 of the heart and the origin of the right innominate artery (fig. 1). 

 This vessel arises at a distance' of 8-5 mm. from the ventricle, 

 and from this point the aorta, reduced in diameter from 4 ram. to 

 3 mm., crosses the thorax, curving round the trachea. The left 

 innominate artery takes origin 6 mm. from the right innominate, 

 and from this point the course of the aorta is upward and back- 

 ward, receiving the obliterated ductus arteriosus 4-5 mm. from 

 the left innominate, and passing above the root of the left lung. 



The left vagus nerve crosses between the left innominate and 

 left precaval vein, the recurrent laryngeal branch turning round 

 the ductus arteriosus. The trunk of the nerve passes on to form 

 anterior and posterior pulmonaiy plexuses in the usual manner. 



The descending thoracic aorta has much the same relation to 

 surrounding structures as in man, lying on the bodies of tlie ver- 

 tebras between the pleural sacs, aud having the thoracic duct and 

 vena azygos major to the right, while the oesophagus lies on its 

 ventral surface. The length from the ductus arteriosus to the 

 opening in the diaphragm is 35 mm. 



The descending thoracic aorta gives off the usual oesophageal 

 and mediastinal branches, aud intercostal arteries to all the spaces 

 below the second. The first space is supplied by a branch from 

 the vertebral, the second space either from the vertebral or, 

 more commonly, from the aorta. The anterior intercostal arteries 

 on the right side pass outv^ards dorsal to the thoracic duct and 

 vena azygos major, below the eighth space they cross on the ventral 

 side. The relations of the sympathetic cord and subcostal muscle 

 are similar to man. 



In one specimen of Pterojjus edulis the upper two spaces on the 

 right side were supplied by the vertebral, the third and fourth by a 

 branch from the innominate (fig. 3, p. 65), the remainder from 

 the aorta. The bronchial arteries in this species arose by a single 

 trunk from the aorta. 



Eight Innominate Artery. — With the exceptions to be presently 

 noted, the right and left innominate arteries correspond closely 

 with regard to their course, distribution, and branches. Springing 

 froui the right extremity of the aortic arch the artery of the right 

 side, 2 mm. in diameter, passes forward and outward, and above 

 (dorsal to) the sterno-clavicular articulation, at a distance of 6 mm. 

 from its origin, divides into the right common carotid and subclavian, 

 the former, much the smaller, appearing like a bi-anch of the main 

 trunk. 



Small mediastinal branches arise from both innominate arteries 

 to supply adjacent structures. 



^ The measurements of arteries aud veins are all taken to the luid-point of 

 the origin of the branch i-eferred to, except when otherwise indicated. 



