DISSECTION OF THE BACK AND THORAX. 101 



of the dorsal spines. In the intercostal space the intercostal artery is 

 accompanied by a vein and nerve, the vein being in front, and the nerve 

 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) anas- 

 tomose 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 

 circumflex iliac arteries. In their descent the intercostal arteries give 

 off costal, pleural, muscular, and cutaneous branches. 



The Intercostal 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 the 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 301 and 333). 



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 analogous 



