INTERCOSTAL NERVES. 343 



Dissection. In a few spaces the internal intercostal muscle may be cut 

 through, and the intercostal nerve and artery traced outwards. 



The INTERCOSTAL NERVES are the anterior primary branches of the 

 dorsal nerves. Twelve in number, they occupy the intercostal spaces, 

 without communicating in a plexus ; and the last is placed below the 

 twelfth rib. The upper six lie between the ribs, and are confined to the 

 wall of the thorax ; whilst the lower six are prolonged into the abdominal 

 wall, where the ribs cease in front. 



Upper six. At first the nerves lie between the pleura and subjacent 

 fascia and the external intercostal muscle, with an artery and vein ; but 

 they enter soon between the intercostals and extend forwards to the middle 

 line of the body. Near the head of the rib each is joined by filaments 

 from the sympathetic. In its course each supplies branches to the inter- 

 costal muscles and the ribs, and cutaneous offsets to the surface ; these are 

 described in the dissections of the upper limb and wall of the abdomen. 



There are some deviations in the first and second nerves from the ar- 

 rangement above specified. 



The first nerve ascends in front of the neck of the highest rib, and ends 

 in the brachial plexus. Before it leaves the chest it supplies to the first 

 intercostal space a branch, which furnishes muscular offsets, and becomes 

 cutaneous by the side of the sternum. There is not any lateral cutaneous 

 offset from this branch, except in those cases in which the second nerve is 

 not as large as usual. 



The second nerve may extend a considerable way on the wall of the 

 chest before entering between the intercostal muscles, and may ascend 

 even to the first space. It is remarkable in having a very large lateral 

 cutaneous branch (p. 225). In front it ends like the others. 



Upper -surface of the diaphragm. The centre of the muscle is tendinous, 

 and the sides are fleshy. In contact with the upper surface are the lungs 

 laterally, and the pericardium in the middle : the phrenic vessels and 

 nerves pierce this surface, external to the pericardium. In the diaphragm 

 are the following apertures : one for the O3sophagus and the pneumogastric 

 nerves, a second for the vena cava, a third for the aorta with the thoracic 

 duct and the vena azygos, and a fourth on each side for the splanchnic 

 nerves. Beneath it the sympathetic passes into the abdomen. 



The arch of the diaphragm towards the thorax has been before referred 

 to (p. 306). 



Directions. The dissector of the thorax now waits while the examina- 

 tion of the Back is made. Afterwards he is to learn the ligaments of the 

 ribs and spine : a notice of these will be found in the following Section. 



SECTION II. 



LIGAMENTS OF THE TRUNK. 



THE ligaments of the vertebras, ribs, and sternum, are included in this 

 Section. 



ARTICULATION OF THE RIBS. The osseous part of each rib is united to 

 the spinal column on the one side, and the rib cartilage on the other, by 

 three sets of ligaments, viz., one between the head of the bone and the 



