DISSECTION OF THE DOG 55 



It leaves the arch about the middle line and passes towards the thoracic inlet 

 immediately ventral to the trachea. The brachio-cephaUc artery gives origin 

 to the left common carotid artery, and shortly afterwards terminates by dividing 

 into the right subclavian and right common carotid arteries. 



A. CAEOTis COMMUNIS SINISTRA.— The left common carotid artery begins 

 ventral to the trachea and travels to the thoracic inlet by running obliquely 

 across the ventral face of the oesophagus. 



A. STJBCLAViA DEXTEA.— The right subclavian artery curves lateralwards 

 to the cranial border of the first rib. Ventral to it are the right innominate 

 vein, the right phrenic, and vagus and sympathetic nerves. The right re- 

 current nerve (a branch of the vagus) bends round the caudal border of the 

 artery. The branches of the right subclavian artery are : (1) Vertebral artery ; 

 (2) costo-cervical trunk ; (3) internal mammary artery ; (4) omo-cervical trunk ; 

 (5) axillary artery. 



A. CAROTis COMMUNIS DEXTEA. — The right common carotid artery passes 

 to the thoracic inlet by crossing the ventral surface of the trachea with a slight 

 degree of obUquity. 



A. SUBCLAVIA SINISTRA. — The left subclavian artery leaves the aortic arch 

 some little distance to the left of the point of origin of the brachio-cephahc 

 artery, and at a more dorsal level. It gains the thoracic inlet by running along 

 a groove formed by the oesophagus and the longus colU muscle. The left 

 phrenic and vagus nerves he ventral to the artery. Its branches are the same 

 as those of the right subclavian artery. 



A. VBETEBEALis. — The vertebral artery is of large size, and on the right 

 side crosses the lateral surface of the trachea to disappear between the scalenus 

 and longus colU muscles, and so reach the foramen in the transverse process 

 of the sixth cervical vertebra. Its subsequent course through the foramina 

 of the rest of the cervical vertebrae will be exposed at a later stage in the 

 dissection. The left vertebral artery crosses the oesophagus instead of the 

 trachea. 



Truncus costo-cervicalis. — The costo-cervical trunk is a large branch 

 of the subclavian artery running towards the vertebral column within the 

 first rib. Its branches are three in number, namely, the deep cervical, supreme 

 intercostal, and transverse cervical arteries. 



A. CERViCALis profunda. — Part of the course of the deep cervical artery 

 has been noted in connection with the serratus ventralis mus(?Ie. There only 

 remains to observe its origin and the initial part of its course round the border 

 of the first rib dorsal to the attachment of the scalenus muscle. 



A. iNTERCOSTALis SUPEBMA. — The Supreme intercostal and transverse 

 cervical arteries generally leave the costo-cervical trunk as a short common 

 vessel. The supreme intercostal runs along under the necks of the second and 

 third ribs, and contributes the second, third, and possibly also the fourth 

 intercostal arteries. 



E 4 



