30 ■ DISSECTION OF THE DOG 



The first eight nerves end a short distance from the sternum by dividing 

 into (1) a ventral cutaneous hrayich (ramus cutaneus ventrahs), and (2) a smaller 

 branch for the transverse thoracic and internal intercostal muscles. The 

 ventral cutaneous ramus becomes superficial about the termination of the 

 external intercostal muscle. 



The last four intercostal nerves are continued beyond the costal arch into 

 the wall of the abdomen, M'here they have aheady been found between the 

 internal oblique and transverse muscles. 



Dissection. — Remove the intercostal muscles from all the interspaces on 

 both sides of the chest from the sternum to about the middle of the length 

 of the interspace. The internal intercostal muscles should be removed 

 with the greatest care, so that the endothoracic fascia between the ribs 

 may be preserved. 



In the region of the sternum a muscle, the transverse thoracic, and an 

 artery and a vein, the internal thoracic or mammary, are exposed on 

 each side. 



A. MAMMAEiA INTERNA. — The internal mammary artery is a branch of 

 the subclavian, and pursues an oblique course in the mediastinum to gain the 

 ventral surface of the transverse thoracic muscle. Here the artery runs along 

 in the region of the joints between the costal cartilages and the sternum, until 

 it reaches the eighth cartilage, where it divides into the cranial epigastric and 

 musctdo-phrenic arteries. 



The following are the collateral branches of the artery : (I) Two inter- 

 costal hranches (rami intercostales) are distributed to the ventral end of 

 each of the first seven intercostal .spaces. These anastomose with the ter- 

 mination of an intercostal artery. (2) Sternal and per/orating branches (rami 

 sternales et perforantes) supply the transverse thoracic and pectoral muscles. 

 (3) Thymic arteries (aa. thymicse). (4) Anterior bronchial arteries (aa. bronchiales 

 anteriores). (5) Anterior mediastinal arteries (aa. mediastinales anteriores. 

 (6) Pericardiaco-phrenic artery (a. pericardiaco-phrenica). With the exception 

 of the first two, these branches are distributed to structures within the 

 thorax, and, consequently, are not accessible at present. 



A. MiTSCXTLO-PHRENiCA. — In Order to examine the termination of the 

 internal mammary artery properly, it may be necessary to cut away a part 

 of the eighth costal cartilage. The musculo-phrenic artery runs along the 

 border of the diaphragm, and is, at first, under cover of the eighth costal 

 cartilage. The artery supplies the diaphragm, and anastomoses with some of 

 the intercostal arteries. 



A. EPiGASTBiCA CEANiALis.— The cranial epigastric artery has already 

 been examined as it runs along the deep face of the straight muscle of the 

 abdomen. A considerable branch has also been noted as becoming superficial, 

 close to the xiphoid process of the sternum. There only remains to examine 



