THE DOR8AL NERVES. 777 



second to the last. Both roots are very slender, the posterior roots only slightly 

 exceeding the anterior in thickness. They gradually increase in length from 

 above downward, and in the lower part of the dorsal region pass down in contact 

 with the spinal cord for a distance equal to the height of at least two vertebrae, 

 before they emerge from the spinal canal. They then join in the intervertebral 

 foramen, and at their exit divide into two primary divisions, a posterior (dorsal) 

 and an anterior (intercostal). 



The first, the second, and the last dorsal nerves are peculiar in some respects. 



Posterior Divisions of the Dorsal Nerves. 



The posterior divisions of the dorsal nerves, which are smaller than the ante- 

 rior, pass backward between the transverse processes, and divide into internal and 

 external branches. 



The internal branches of the six upper nerves pass inward between the Semi- 

 spinalis dorsi and Multifidus spinae muscles, which they supply, and then, piercing 

 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 posterior primary divisions of the dorsai nerves 

 are twelve in number. The six upper cutaneous nerves are derived from the in- 

 ternal 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 fre- 

 quently 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 buttock. 



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. It frequently receives a connecting twig from the second 

 dorsal nerve, which passes upward over the neck of the second rib. 



