DISSECTION OF THE BAOK AND THORAX. 123 



The Triangularis Stbrni (Fig. 7). This muscle arises from the 

 lateral margin of the thoracic surface of the sternum, beginning at a 

 point opposite the 2nd costal cartilage, and extending backwards to the 

 ensiform cartilage, from the edge of which the last few fibres arise. It 

 is inserted into the costal cartilages from the 2nd to the 8th inclusive, 

 and into an aponeurosis on the internal intercostal muscles. Its outer 

 edge is strongly serrated. It covers the internal thoracic vessels, and is 

 lined by pleura on its upper face. 



Action. — It pulls the cartilages to which it is attached inwards and 

 backwards, and thus assists in expiration. 



The Internal Thoracic (Mammary) Artery (Fig. 7). This vessel, 

 detached from the axillary artery at the 1st rib, descends on the inner 

 face of that bone, and disappears beneath the triangularis sterni muscle. 

 When the muscle is removed, the artery is seen to pass backwards at 

 the edge of the sternum, crossing the chondro-sternal joints. Over or 

 about the 8th of these joints it divides into the asternal and anterior 

 abdominal arteries. The asternal branch emerges from under cover of 

 the triangularis sterni, and runs up the cartilage of the 9th rib, on the 

 thoracic side of the origin of the diaphragm. About the upper end of 

 the cartilage it passes through the edge of the diaphragm to its 

 abdominal side. The anterior abdominal artery dips down between the 

 cartilage of the 9th rib and the edge of the ensiform cartilage, and 

 enters the abdominal wall. The collateral branches of the internal 

 thoracic are : — 



1. Branches to the mediastinum and pericardium. 



2. Pectoral branches, perforating the intercostal space and anasto- 

 mosing with the external thoracic artery. 



3. Intercostal branches, which ascend to anastomose with the inter- 

 costal arteries. This series is continued by branches of the asternal 

 artery. 



The Internal Thoracic Vein runs in company with the artery, 

 and internal to it. Beneath the triangularis sterni it is placed 

 between the artery and the fibrous cord that traverse the edge of the 

 sternum. 



Suprasternal Lymphatic Glands. These include (1) a group of 

 glands on the thoracic side of the insertion of the diaphragm across the 

 ensiform cartilage, and (2) some small scattered glands along the course 

 of the internal thoracic vessels, 



EXAMINATION OF THE LUNG. 



Physical Characters.— The exterior of the lung is exquisitely smooth 

 in virtue of its pleural covering. Through this thin, transparent cover- 

 ing, the surface is seen, especially when the lung is distended, to be 



