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THE NERVOUS SYSTEM 



2. THE THORACIC NERVES 



The anterior primary divisions of the thoracic nerves, with the exception of 

 the first, retain, in the simplest form, the characters of anterior primary divisions 

 of the typical spinal nerve. They do not form plexuses, but remain distinct from 

 each other. Each divides into an easily recognisable lateral or dorsal and anterior 

 or ventral branch (figs. 762 and 763), and they are not distributed to the limbs. 

 The first, second, and last thoracic nerves, on account of their pecuharities, require 

 separate description. The remainder are separable into two groups, an upper and 

 a lower. The upper group consists of four nerves, the third to the sixth inclusive, 

 which are distributed entirely to the thoracic wall. The lower group contains 

 five nerves, the seventh to the eleventh inclusive, which are distributed partly 



Fig. 762. — Diagram of the Distribution of a Typical Thoracic Nerve. 



Longissimus dorsi. 



Semispinalis dorsi 



Medial branch' r 



Superior costo-transverse |i 



ligament p 



Dorsal root-T" 



Ventral root ~ 

 Recurrent branch 

 Sympathetic ganglion 



Viceral branch ^pjjK 

 Branch to aorta 

 (Esophagus 



Internal mammary artery 

 Transverse thoracic muscle 



Sternum 



Ilio-costalis[.dorsi 

 Lateral branch 



^Posterior primary division 



Anterior primary division 

 Internal intercostal muscle 

 External intercostal muscle 



Lateral cutaneous branch 



Anterior branch 



Anterior intercostal membrane 



to the thoracic and partly to the abdominal wall. The upper group is therefore 

 purely thoracic in distribution, and the lower thoraco-abdominal. 



The first thoracic nerve is connected with the first thoracic sympathetic gang- 

 lion, and it frequently is joined by a small branch with the second nerve. It is 

 distributed chiefly to the upper limb. Opposite the superior costo-transverse 

 ligament of the second rib it divides into a larger and a smaller branch; the 

 larger passes upward and lateralward, between the apex of the pleura and the 

 neck of the first rib, and on the lateral side of the superior intercostal artery, to 

 the root of the neck, where it joins the brachial plexus. The smaller branch con- 

 tinues along the intercostal space, below the first rib and between the intercostal 

 muscles in which, as a rule, all its fibres terminate. 



However, the smaller branch may give off a lateral cutaneous branch which connects with the 

 medial brachial (lesser internal) cutaneous nerve and with the intercosto-braciiial nerve in the 

 axillary fossa; and occasionally it terminates in an anterior cutaneous branch at the anterior 

 extremity of the first intercostal space. 



The second thoracic nerve, as it lies between the pleura and the superior 

 costo-transvcrsc! ligaincut of tin; third rib, gives a branch to the first nerve, then 

 it j)ierces tho. posterior intercostal membrane and passes between the external and 

 internal intercostal muscles in the second intercostal space. In the dorsal part 

 of the space it sends branches backward, through the external intercostal muscle, 



