488 



DISSECTION OF THE THOBAX. 



smallest 

 splanchnic 

 to renal 

 plexus. 



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 

 accompanies the other nerves through the diaphragm ; in the 

 abdomen it ends in the renal plexus. This nerve may be absent, 

 and its place is then taken by an offset of the preceding. 



Soft parts 

 bounding 

 the thorax. 



Subcosta s \ 

 position ; 



attach- 

 ments ; 



irregulari- 

 ties ; 



and use. 



Intercostal 

 muscles. 



Inner layer 

 reaches 

 angle of 

 the rib; 



relations. 

 Outer layer 



extends 

 back to 

 tuberosity 

 of the rib. 



Trace 



Eleven 



intercostal 



nerves. 



Upper and 

 lower ones 

 differ. 



Last dorsal 

 nerve. 



PARIETES OF THE THORAX. 



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

 with the intervening nerves and arteries ; and inside the ribs is a 

 thin fleshy layer at the back, the subcostal muscles. The base 

 of the thorax is formed by the diaphragm. 



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

 situate on the inner surface of the ribs, where the internal inter- 

 costals cease. Apparently part of the inner intercostal s, 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. The subcostals draw together, and depress the ribs, thus 

 acting as expiratory muscles. 



INTERCOSTAL MUSCLES. The anterior part of these muscles has 

 been described (p. 438) ; and the posterior part may now be examined 

 from the inner side. 



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

 angles of the ribs, or somewhat farther in the upper spaces. Where 

 the fibres cease, a thin fascia (posterior intercostal aponeurosis) is 

 continued inwards 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 subcostal 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. While this muscle 

 extends backwards to the tuberosity of the rib, it is generally 

 absent, as already described, in front, between the rib-cartilages. 



Dissection. In a few spaces the internal intercostal muscle 

 may be cut through, and the intercostal nerve and artery traced 

 outwards. 



The INTERCOSTAL NERVES, eleven in number, are anterior primary 

 branches of dorsal nerves ; and they pass from the intervertebral 

 foramina into the intercostal spaces without forming a plexus. 

 Near the head of the rib each nerve is joined to the sympathetic 

 by the two communicating filaments just mentioned. The upper 

 six are confined to the wall of the thorax ; but the lower five are 

 prolonged into the abdominal wall, where the ribs cease in front. 



The anterior branch of the twelfth dorsal nerve lies below the 

 last rib, and is seen in the dissection of the abdomen. 



