THORACIC WALL 9 



Musculi Intercostales Interni. The internal intercostal 

 muscles, laid bare by the dissection described, will be seen 

 to be similar in their constitution to the external muscles. 

 The fibres, however, run in the opposite direction viz., from 

 above, obliquely downwards and backwards. Superiorly, each 

 is attached to the inner surface of the upper rib, immediately 

 above the costal groove ; inferiorly, it is attached to the 

 inner surface of the lower rib, close to the upper margin. 

 The internal intercostal muscles are prolonged forwards to the 

 sternum. Posteriorly, they extend to the angles of the ribs. 

 The posterior intercostal membranes extend from the vertebral 

 column to the posterior borders of the internal intercostals, 

 where they become continuous with the fascial layer between 

 the external and internal intercostal muscles. They will be 

 seen when the thorax is opened. 



If the internal oblique muscle of the abdomen has not been removed, 

 the dissector should note that the anterior fibres of the lowest two internal 

 intercostal muscles become continuous with the fibres of that muscle. 



The lateral and anterior cutaneous branches of the inter- 

 costal nerves have already been found, but the main parts of 

 the trunks of the nerves are concealed under cover of the 

 lower borders of the ribs, and a little dissection is necessary to 

 expose them and the intercostal arteries and veins which lie, 

 at still higher levels, under cover of the ribs. 



Dissection. The intercostal nerves and vessels should be 

 dissected in two or three spaces. If the arteries are not in- 

 jected it may be difficult or impossible to display them in the 

 anterior parts of the spaces, but the posterior parts of the inter- 

 costal branches of the aorta and of the subclavian artery will easily 

 be found later, after the thorax is opened (see pp. 33-35). 

 First find the lateral cutaneous branch of an intercostal nerve, 

 preferably that of the third, fourth, or fifth intercostal space. 

 It will serve as a guide to the trunk of the nerve. Follow the 

 lateral cutaneous branch to the lower border of the rib which 

 bounds the space above, then take the bone forceps and cut 

 away the lower border of the rib until the origin of the lateral 

 cutaneous branch from the trunk of the nerve is found. When 

 the trunk of the intercostal nerve has been secured, follow it 

 backwards as far as possible, removing the lower part of the 

 rib which covers it, and, at the same time, clean the intercostal 

 artery and vein which lie above it, if they can be found. Next 

 follow the trunk of the nerve forwards beyond the origin of the 

 lateral cutaneous branch. About midway between the vertebral 

 column and the sternum it leaves the artery and enters the 

 substance of the internal intercostal muscle, whilst the artery 

 continues forwards on the internal intercostal muscle to 

 anastomose with an intercostal branch of the internal mammary 



