DISSECTION OF THE BACK AND THORAX. 121 



veins of the right side. Its opening is provided with a valve. It 

 discharges into the venous system the lymph collected in the right 

 anterior half of the animal, viz., the right fore limb, and the right 

 side of the head, neck, chest-wall, and diaphragm. 



Directions. — Cut out the trachea and lungs with the heart and great 

 vessels. Sever the lungs from the heart by cutting the great vessels at 

 the root, and set both lungs and heart aside in carbolic or other preser- 

 vative solution to serve in the examination of the structiu-e of these 

 organs. Or, since they are likely to be much decomposed, it will be 

 better to discard them if fresh organs can be obtained. In the mean- 

 time separate the sternum and costal cartilages as shown in Fig. 7, 

 and dissect the triangvilaris sterni muscle and the internal thoracic 

 vessels. Portions of the longus colli and psoas muscles which are 

 attached to the lower face of the dorsal vertebrse should be noticed. 

 The longus colli is described at page 156, and the psoas muscles at 

 page 325. 



The Triangularis Sterni (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- 



