THE ASTERIOR VENA CAVA. 687 



extremity of its inflection crosses the oesophagus und trachea to the right, and 

 is included between these two tubes on the one side, and the right hiyer 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, aa 

 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 (Figs. 386, 389). 



The jugular is a satellite vein of the carotid artery. 



Origin. — It commences behind the inferior maxilla, below the articulation 

 of the jaw, by two large roots— the superficial temporal vein and the internal 

 maxillary vein., which correspond to the two terminal branches of the external 

 carotid artery. 



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

 at first in the substance of the parotid gland, afterwards in the muscular inter- 

 space designated the jugular furrow, which is comprised between the adjacent 

 1 (orders of the mastoido-humeralis and sterno-maxillaris muscles. Reaching the 

 inferior extremity of the neck, it terminates in the following manner : — 



Tei'mination. — On arriving near the entrance to the chest, the two jugulars 

 unite in forming a vessel named the confluent of the jvgidars. This confluent, 

 into which open the two axillary veins, is comprised between the two first ribs, 

 and situated below tlie trachea, in the middle of the prepectoral lymphatic glands. 

 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 entrance into 

 the circulation when the jugular or axillaiy veins are opened, as well as affording 

 an indication how to prevent this serious accident. 



Relations. — 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 cervical panniculus, 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 situations, above or below : above, it is related 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. 8. Occipital vein. 4. Svhmaxillary or facial vein. 

 5. Thyroid vein. 6. Cephalic vein. 7. Parotideal and innominate muscular 

 branches. 



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

 branches of the artery of the same name, and entering the jugular close to its 

 origin, either separately or after forming a common trunk. 



