846 THE NERVOUS SYSTEM. 



the origins of the Rhomboidei and Trapezius muscles, become cutaneous by the 

 side of the spinous processes and ramify in the integument. The internal branches 

 of the six lower nerves are distributed to the Multifidus spinae, without giving off 

 any cutaneous filaments. 



The external branches increase in size from above downward. They pass 

 through the Longissimus dorsi to the cellular interval between it and the Ilio- 

 costalis, and supply those muscles, as well as their continuations upward to the 

 head, and the Levatores costarum ; the five or six lower nerves also give off 

 cutaneous filaments, which pierce the Serratus posticus inferior and Latissimus dorsi 

 in a line with the angles of the ribs, and then ramify in the integument. 



The cutaneous branches of the dorsal nerves are twelve in number. The six 

 upper cutaneous nerves are derived from the internal branches of the posterior 

 divisions of the dorsal nerves. They pierce the origins of the Rhomboidei and 

 Trapezius muscles, and become cutaneous by the side of the spinous processes, 

 and then ramify in the integument. They are frequently furnished with gangliform 

 enlargements. The six lower cutaneous nerves are derived from the external 

 branches of the posterior divisions of the dorsal nerves. They pierce the Serratus 

 posticus inferior and Latissimus dorsi in a line with the angles of the ribs, and 

 then ramify in the integument. 



Anterior Divisions of the Dorsal Nerves. 



The anterior divisions of the dorsal nerves (intercostal nerves) are twelve in 

 number on each side. They are, for the most part, distributed to the parietes of 

 the thorax and abdomen, separately from each other, without being joined in a 

 plexus ; in which respect they differ from the other spinal nerves. Each nerve is 

 connected with the adjoining ganglia of the sympathetic by one or two filaments. 

 The intercostal nerves may be divided into two sets, from the difference they 

 present in their distribution. The six upper, with the exception of the first and 

 the intercosto-humeral branch of the second, are limited in their distribution to the 

 parietes of the chest. The six lower supply the parietes of the chest and abdomen, 

 the last one sending a cutaneous filament to the hip. 



The First Dorsal Nerve. The anterior division of the first dorsal nerve divides 

 into two branches : one, the larger, leaves the thorax in front of the neck of the 

 first rib, and enters into the formation of the brachial plexus ; the other and 

 smaller branch runs along the first intercostal space, forming the first intercostal 

 nerve, and terminates on the front of the chest by forming the first anterior 

 cutaneous nerve of the thorax. Occasionally this anterior cutaneous branch is 

 wanting. The first intercostal nerve, as a rule, gives off no lateral cutaneous 

 branch, but sometimes a small branch is given off which communicates with the 

 intercosto-humeral. 



The Upper Dorsal Nerves. The anterior divisions of the second, third, fourth, 

 fifth, and sixth dorsal nerves and the small branch from the first dorsal are 

 confined to the parietes of the thorax, and are named upper or pectoral intercostal 

 nerves. They pass forward in the intercostal spaces with the intercostal vessels, 

 being situated below them. At the back of the chest they lie between the pleura 

 and the External intercostal muscle, but are soon placed between the tAvo planes 

 of Intercostal muscles as far as the middle of the rib. They then enter the 

 substance of the Internal intercostal muscles, and, running amidst their fibres as far 

 as the costal cartilages, they gain the inner surface of the muscles and lie between 

 them and the pleura. Near the sternum they cross the internal mammary artery 

 and Triangularis sterni muscle, pierce the Internal intercostal and Pectoralis major 

 muscles, and supply the integument of the front of the chest and over the 

 mammary gland, forming the anterior cutaneous nerves of the thorax, the branch 

 from the second nerve becoming joined with the supraclavicular nerves of the 

 cervical plexus. 



Branches. Numerous slender muscular filaments supply the Intercostals, the 

 Infracostales, the Levatores costarum, Serratus posticus superior, and Triangularis 



