THE THORACIC NERVES. 1315 



the area immediately above Poupart's ligament and the pubes being innervated, not 

 by the thoracic, but by the lumbar nerves (Fig. 1105). The supply of the cutane- 

 ous area is provided by two rows of sensory twigs, which become superficial by 

 piercing the musculature and deep fascia of the trunk. Each of the thoracic nerves, 

 with the exception of the first, sends out a lateral cutaneous branch and, with no 

 exceptions, an anterior cutaneous branch. The upper thoracic nerves deviate 

 variously from this typical arrangement, the first having no lateral and sometimes no 

 anterior cutaneous branch, and a portion of the lateral cutaneous branch of the 

 second, called the intercosto-humeral nerve, leaving the thorax to be distributed 

 in the upper extremity. The third nerve of the series is the first to present 

 a typical arrangement, although it, indeed, sometimes forms a loop with the 

 lesser internal cutaneous nerve of the arm. The anterior cutaneous branches are 

 the terminal portions of the thoracic nerves and are constant in their arrangement 

 and distribution, with the exception of the first, which is either very small or absent 

 and a filament from the last, which passes over the crest of the ilium to the 

 gluteal integument. 



After separating from the posterior primary divisions, the anterior primary 

 divisions of the thoracic nerves, with the exception of the twelfth, enter the inter- 

 costal spaces by passing between the anterior costo-transverse ligaments and the 

 external intercostal muscles. From this situation to the angles of the ribs they lie 

 between the posterior intercostal membrane and the external intercostal muscles. 

 Anterior to this point, they are situated between the two sets of intercostal muscles, 

 as far forward as the termination of the external set of muscles at the. costo-chondral 

 articulations, from which point forward their superficial covering is the anterior inter- 

 costal membrane and the deep the internal intercostal muscles. At first they lie within 

 the upper part of the intercostal space, but as they advance they show a tendency 

 to occupy the middle of the space. While accompanying the intercostal vessels, they 

 lie below the latter and at a greater distance from the rib next above. The upper 

 two nerves extend for a portion of their course along the inner surface of the corre- 

 sponding ribs; the twelfth passes in front of the quadratus lumborum. 



The upper thoracic nerves, as they approach the margin of the sternum, tra- 

 verse the substance of the internal intercostal muscles and hold a position anterior 

 to the internal mammary artery and the lateral portion of the triangularis sterni 

 muscle. They terminate by piercing the anterior intercostal membrane and the pec- 

 toralis major, and ramify in the pectoral integument as the anterior cutaneous nerves 

 of the thorax (Fig. 1105). 



The lower thoracic nerves pass forward and at the anterior ends of the ribs 

 take up a deeper position in the trunk wall by piercing the substance of the internal 

 intercostal muscles. They then traverse the intervals between the digitations of 

 the diaphragm and enter the abdominal wall, the seventh, eighth and ninth nerves 

 lying behind the cartilages of the eighth, ninth and tenth ribs respectively. From 

 this point their course is ventral, between the internal oblique and the transversalis, 

 as far as the lateral edge of the rectus sheath, which they enter by piercing its pos- 

 terior lamella. They ultimately turn forward and become superficial by traversing 

 the rectus and its anterior aponeurotic covering, terminating as the anterior cuta?ieous 

 nerves of the abdomen (Fig. 1105). 



Communications. Each thoracic nerve is connected with the sympathetic 

 gangliated cord by one or two rami communicantes (Fig. 1130). Ordinarily there 

 is no intercommunication between the upper intercostal nerves, but in rare instances 

 a twig passes from one nerve over the inner surface of the rib next below to the sub- 

 jacent nerve. The lower three or four thoracic nerves, while lying between the broad 

 abdominal muscles are occasionally united to one another, sometimes to the extent 

 of forming a small plexus. 



Peculiar thoracic nerves. The first, second, twelfth, and sometimes the 

 third, thoracic nerves present peculiarities which differentiate them from the others. 



The first thoracic nerve sends a large portion of its fibres to the brach- 

 ial plexus, thus suffering great reduction in its size. Although occasionally a very 

 small branch to the axilla is found, a lateral cutaneous branch is rare, it being 

 generally held that the contribution of this nerve to the brachial plexus is the 



