342 DISSECTION OF THE THORAX. 



From the third and fourth ganglia offsets are sent to the posterior pulmo- 

 nary plexus. 



The branches of the lower six are larger and much whiter than the 

 others, and are united to form visceral or splanchnic nerves of the abdo- 

 men : these are three in number (large, small, and smallest), and pierce 

 the diaphragm to end in the solar and renal plexuses. 



The great splanchnic nerve is a large white cord, which receives roots 

 apparently from only four or five ganglia (sixth to the tenth), but its fibres 

 may be traced upwards on the knotted cord as high as the third ganglion. 

 Descending on the bodies of the vertebrae, it pierces the fibres of the crus 

 of the diaphragm, and ends in the semilunar ganglion of the abdomen. 

 At the lower part of the thorax the nerve may present a ganglion. 



The small splanchnic nerve begins in the tenth and eleventh ganglia, 

 or in the intervening cord. It is transmitted interiorly through the crus 

 of the diaphragm, and enters the part of the solar plexus by the side of 

 the coeliac artery. 



The smallest splanchnic nerve springs from the last ganglion, and ac- 

 companies the other nerves through the diaphragm : in the abdomen it 

 ends in the renal plexus. This nerve may be absent, and its place may 

 be taken by an offset of the preceding. 



PARIETES OF THE THORAX. 



Between the ribs are lodged the two layers of intercostal muscles, with the 

 intervening nerves and arteries ; and inside them lies a thin fleshy layer 

 behind, the infracostals. At the base of the thorax is the diaphragm. 



The INFRACOSTAL MUSCLES are small slips of fleshy fibres, which are 

 situate on the inner surface of the ribs, where the internal intercostals 

 cease. Apparently part of the inner intercostals, they arise from the 

 inner surface of one rib, and are attached to the like surface of the rib 

 next succeeding. 



They are uncertain in number, but there may be ten : they are smaller 

 above than below, and the upper and lower may pass over more than one 

 space. 



Action. These thin muscles approach the ribs to one another, dimin- 

 ishing the size of the thoracic cavity, and act thus as expiratory muscles. 



INTERCOSTAL MUSCLES. The anterior part of the muscles has been 

 described (p. 237) ; and the posterior part may be now examined from 

 the inner side. 



The inner muscle begins at the sternum, and reaches backwards to the 

 angle of the ribs in the middle spaces, but, higher and lower, the muscu- 

 lar fibres approach nearer the spine. Where the fibres cease, a thin fascia 

 is continued backwards over the outer muscle. The inner surface is lined 

 by the pleura, and the opposite surface is in contact with the intercostal 

 nerve and vessels. 



External muscle. When the fascia and the infracostal muscles have 

 been removed, the external intercostal will be seen between the posterior 

 border of the internal muscle and the spine. Its fibres cross those of the 

 inner intercostal layer. Whilst this muscle extends backwards to the 

 tubercle of the rib, it does not reach further forwards than the rib carti- 

 lages, as before said. 



Action. The use of the intercostal muscles in respiration is given in 

 p. 238. 



