DISSECTION OF THE BACK AND THORAX. 99 



posterior. In the upper third of the space the vessels descend between 

 the outer and inner muscles, and rest in the groove at the posterior edge 

 of the rib. For the rest of their course they are under cover of the 

 hinder edge of the rib, and, generally, between the inner muscle and the 

 pleura ; but, here and there, slips of the inner muscle may pass between 

 the vessels and the pleura. At the lower extremities of the intercostal 

 spaces the arteries behave as follows : — The first six (or seven) anastomose 

 with ascending branches from the internal thoracic artery ; the remainder 

 as far as the thirteenth anastomose with similar branches from the 

 asternal artery ; and the last four run into the abdominal wall and are 

 expended in its muscles, anastomosing with the abdominal and ciroumilex 

 iliac arteries. In their descent the intercostal arteries give off costal, 

 pleural, muscular, and cutaneous branches. 



The Inteecostal Veins accompany the arteries. On the left side 

 the first joins the superior cervical vein, the next ten or eleven join 

 the left dorsal vein, and the last five or six the great vena azygos. 

 On the right side the first joins the superior cervical vein, the next 

 three join the dorsal vein, and the remaining thirteen the great vena 

 azygos. 



The Dorsal Nerves. There are eighteen dorsal nerves, one emerging 

 by the intervertebral foramen behind each dorsal vertebra. Each divides 

 in the foramen to form a superior and an inferior primary branch. The 

 superior primary branch supplies the muscles in the costo-vertebral 

 groove, and the superjacent skin. The inferior primary branch of the 

 1st nerve, after detaching a very slender intercostal twig, joins the 

 brachial plexus. The 2nd nerve gives a slender branch to the brachial 

 plexus, and is continued as the intercostal nerve of the second space. 

 The inferior primary branches of the succeeding nerves, except the last, 

 are directly continued as intercostal nerves. The inferior primary 

 branch of the last (18th) dorsal nerve descends behind the last rib (see 

 pages 292 and 324). 



The Intercostal Nerves. — These accompany the intercostal vessels, and 

 terminate thus : — The 1st intercostal nerve is very slender and does not 

 reach the bottom of the space ; the six nerves behind the 1st perforate 

 the pectoral muscles and become cutaneous at the side of the sternum ; 

 the others (ten) are continued beyond the lower extremities of the inter- 

 costal spaces to be distributed in the abdominal wall. 



The intercostal nerves give branches to the muscles of the same name, 

 and about the middle of the intercostal space each gives off a large per- 

 forating branch (lateral cutaneous of man) to supply the panniculus and 

 overlying skin. 



The Lumbar Nerves and Vessels. The superior primary branches of 

 these nerves (six in number) have a distribution in the loins analagous 

 to the corresponding branches of the dorsal nerves in the back. They 



