DISSECTION OF THE DOG 31 



its origin as one of the two terminal branches of the internal mammary artery, and 

 to note that it gains the abdominal wall by piercing the edge of the diaphragm. 



M. transverstjs thoracis. — The transverse muscle of the thorax arises 

 from the third or fourth to the seventh segments of the sternum and from the 

 ninth costal cartilage. Its insertion is to the ribs and their cartilages from 

 the second or third to the seventh or eighth, and to the endothoracic fascia 

 of the intervals between the ribs. 



Cavtjm thoracis. — It is necessary that the dissector should have, at least, 

 some general idea of the thoracic cavity and the organs contained therein 

 before he proceeds with their examination. The thorax is a cavity with a 

 bony wall formed by the thoracic vertebrae, the ribs, and the sternum. It 

 resembles a laterally flattened cone, with a sloping base bounded by the 

 diaphragm, which forms the thin, muscular partition between the thorax and 

 the abdomen. The diaphragm being markedly concave when viewed from the 

 abdominal side, it is important to remember that the cavity of the thorax is 

 not so spacious as an examination of the skeleton would lead one to imagine. 



Between the first pair of ribs, i.e. at the apex of the cone, numerous structures 

 pass to and from the neck. The chief organs contained within the chest are 

 the heart and the two lungs. The heart lies between the lungs and is enclosed 

 in a fibro-serous sac, the 'pericardium. The lungs are lateral to the heart and 

 fill the greater part of the thoracic cavity. Except where it is connected with 

 the heart by large blood-vessels, and where it is joined to the trachea by the 

 bronchus, each lung lies free in its own side of the thorax. A serous membrane, 

 the pleura, covers each lung and lines the corresponding part of the thoracic 

 wall. Where the two pleurae come into contact with, or approach each other 

 in, or near the median plane, they form the mediastinal septum (septum 

 luediastinale), a partition in which all the thoracic contents, with the exception 

 of the lungs, are contained. Part of the septum and some features of the 

 pleural cavity must now be displayed. That part of the pleura which covers 

 the lungs is known as visceral : the rest of the membrane is the parietal pleura. 



Dissection. — Carefully free the pleura from the ribs from the second to 

 the seventh, and divide these ribs about the middle. Cut through the 

 sternum between the first and second and between the seventh and eighth 

 costal cartilages. Remove the sternum and portions of ribs so isolated. 



The detached part of the sternum and the costal cartilages articulating 

 therewith must be laid aside for the subsequent examination of the joints. 



The dissection leaves the pleural sacs intact, with the costal pleura (that 

 part of the pleura originally attached to the ribs) lying loose upon 

 the lungs. The interior of the pleural cavities should now be exposed 

 by making an incision through the costal pleura on each side close to 

 the cut ends of the ribs, and a second incision at right angles to the first. 



Pleura. — The disposition of the pleura must now be examined. Two 



