TSE ANTMEIOB VENA CAVA. 601 



aorta.* The terminal extremity of its inflection crosses the cssophagus and 

 trachea to the right, and is included between these two tubes on the one 

 side, and the right layer of the mediastinum on the other. 



Its roots are some branches emerging from the spinal and psoas muscles, 

 and which are not usually in direct communication with the posterior vena 

 cava, as in Man and the other animals. 



During its progress, it receives the first lumbar and the satellite veins 

 of all the aortic intercostal arteries, right and left. But when the small 

 vena azygos is prolonged backwards beyond the posterior extremity of the 

 subcostal artery, that vessel, as we have already seen, forms the confluent of 

 a certain number of left posterior intercostals 



JUGULAR VEINS. 



The jugular is a satellite vein of the carotid artery. 



Origin. — It commences behind the inferior maxilla, below the articula- 

 tion of the jaw, by two largo roots : the superficial temporal trunk and the 

 internal maxillary vein, which correspond to the two terminal branches of 

 the external carotid artery (Fig. 290). 



Situation — Direction. — This vessel passes downward and backward, 

 lodged at flrst in the substance of the parotid gland, afterwards in the 

 muscular interstice designated the jugular channel, and which is comprised 

 bet\Yeen the adjacent borders of the levator humeri (mastoido-humeralis) and 

 sterno-maxillaris- muscles. Beaching the inferior extremity of the neck, it 

 terminates in the following manner (Fig. 290) : 



Termination. — On arriving near the entrance to the chest, the two 

 jugulars unite in forming a vessel named the confluent of the jugulars. This 

 confluent, into the sides of which open the two axillary veins, is comprised 

 between the two first ribs, and situated below the trachea, in the middle of the 

 lymphatic glands at the opening of the cliest. Fixed by fibrous bands to the 

 neighbouring parts, and particularly to the two first ribs, the walls of the 

 jugular confluent do not collapse when the venous system is in a state of 

 vacuity : an anatomical peculiarity which it is necessary to understand, in 

 order to explain the manner in which air obtains an entrance into the circu- 

 latory system when the jugular or axillary veins are opened, as well as 

 affording an indication how to prevent this serious accident. 



Belations. — At its upper extremity, the jugular vein is surrounded by the 

 parotideal tissue. For the remainder of its extent, it is covered externally 

 by the subcutaneous muscle of the neck, and by the branches of the cervical 

 plexus which creep on the external surface of that muscle. Inwardly, 

 its relations vary as we consider its situation, above or below : above, it 

 responds to the subscapulo-hyoideus muscle, which separates it from the 

 common carotid, and its satellite nerves ; in its inferior moiety, it is in 

 direct relation with that vessel, which is above it, as well as with the trachea, 

 and even, though only on the left side, with the oesophagus. 



Collateral Affluent Vessels. — The collateral veins which go to the 

 jugular from its origin to its termination, are : 1, Maxillo-muscular veins; 

 2, Posterior auricular vein ; 3, Occipital vein ; 4, External maxillary, or glosso- 

 facial vein; 5, Thyroid vein; 6, Cephalic vein; 7, Parotideal and innominate 

 muscular branches. 



A. Maxillo-musculak Veins. — Two in number, corresponding to the 



' Sometimes the azygos is situated between the aorta and the thoracic duct. When 

 the latter lies to the left side, it is in direct contact with the posterior aorta. 



