DORSAL INTERCOSTAL. 647 



DORSAL NERVES. 



The Dorsal Nerves are twelve in number on each side. The first appears 

 between the first and second dorsal vertebrae, and the last between the last dorsal 

 and first lumbar. 



The roots of origin of the dorsal nerves are few in number, of small size, and 

 vary but slightly from the second to the last. Both roots are very slender ; the 

 posterior roots only slightly exceeding the anterior in thickness. These roots 

 gradually increase in length from above downwards, and remain in contact with 

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

 the lower part of the dorsal region. They then join in the intervertebral fora- 

 men, and, at their exit, divide into two branches, a posterior, or dorsal, and an 

 anterior, or intercostal branch. 



The first and last dorsal nerves are peculiar in several respects (see next page). 



The posterior branches of the dorsal nerves, which are smaller than the inter- 

 costal, pass backwards between the transverse processes, and divide into external 

 and internal branches. 



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

 through the Longissimus Dorsi, corresponding to the cellular interval between 

 it and the Sacro-lurnbalis, and supply those muscles, as well as their continua- 

 tions upwards to the head, and the Levatores Costarum; the five or six lower 

 nerves also give off cutaneous filaments. 



The internal branches of the six upper nerves pass inwards to the interval 

 between the Multifidus Spinae, and Semispinalis Dorsi muscles, which they 

 supply ; and then, piercing the origin of the Khomboidei and Trapezius, become 

 cutaneous by the side of the spinous processes. The internal branches of the 

 six lower nerves are distributed to the Multifidus Spinae, without giving off any 

 cutaneous filaments. 



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

 upper being derived from the internal branches, and the six lower from the 

 external branches. The former pierce the Rhomboid and Trapezius muscles, 

 close to the spinous processes, and ramify in the integument. They are fre- 

 quently furnished with gangliform enlargements. The six lower cutaneous 

 branches pierce the Serratus Posticus Inferior, and Latissimus Dorsi, in a line 

 with the angles of the ribs. 



INTERCOSTAL NERVES. 



The Intercostal Nerves (anterior branches of the dorsal nerves), are twelve 

 in number on each side. They are 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, are limited 

 in their distribution to the parietes of the chest. The six lower supply the 

 parietes of the chest and abdomen. 



The Upper Intercostal Nerves pass forwards 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 two planes of Intercostal muscles as far as the costal cartilages, 

 where they lie between the pleura and the Internal Intercostal muscles. Near 

 the sternum, they cross the internal mammary artery, and Triangularis Sterni, 

 pierce the Internal Intercostal and Pectoralis Major muscles, and supply the 

 integument of the mamrna and front of the chest, forming the anterior cuta- 

 neous nerves of the thorax ; the branch from the second nerve becoming 

 joined with the clavicular nerve. 



