THE PLEURA. 309 



Reflect the skin across the ventrimesou as far as the transverse 

 incision extends. Scrape or dissect the muscles from the ectal sur- 

 face of the ribs and cartilages near their union ; see Fig. 50. 



C. With the arthrotome or strong scissors, cut through the tho- 

 racic wall on both sides, just mesad of the junction of the cartilages 

 and ribs. 



806. Pleura, Septum mediastinale. Grasp the cut edges of 

 the right costicartilages and turn the sternum partly to the left side. 

 The ental surface of the thoracic wall is covered by a smooth glis- 

 tening membrane, the pleura, a serous membrane like the perito- 

 neum ( 725) ; like the latter, it may be detached over a small space 

 by means of the tracer. 



At the meson will be seen a transparent curtain containing 

 blood vessels and more or less fat. This is the mediastinum, me- 

 diastincil septum or septum thoracis (Fig. 7, 99, 100). It divides 

 the thorax into a right and left half. Each half of the thorax is 

 lined by a separate serous sac, and the meeting of these on the 

 meson produces the mediastinal septum. The thorax thus differs 

 markedly from the abdomen, where there is but one serous sac 

 ( 725), but the thoracic organs, like those of the abdomen, are all 

 properly ectad of or outside the serous membrane. Some of the 

 organs, as the heart, are between the two walls of the septum, while 

 others, as the lungs, are apparently within the sacs, as the alimen- 

 tary canal is within the peritoneal sac (see Fig. 78). 



807. After the mediastinal septum has been examined, make, 

 with bone scissors, a transverse incision from the right to the left 

 between the incisions in the thoracic wall, cutting the soft parts in 

 the intercostal space between the 9th and 10th ribs. Then cut the 

 mediastinum near its attachment to the sternum to a point opposite 

 the 4th costal cartilage, avoiding injury to the blood vessels. 

 Finally, turn the sternum cephalad and secure it in this position 

 with a string or a pin. 



With the nippers, cut the ribs, excepting the first, on each side 

 along a line about 3 cm. from their tubercula, and with scissors or 

 a scalpel, cut the soft parts and remove the freed portion of the 

 thoracic wall. 



808. Thymus gland. If the cat is young, there will be seen 

 an elongated pinkish body (Fig. 77) extending along the ventral 

 side of the trachea and great vessels from the heart to a point some- 

 what cephalad of the first rib. This is one of the so called ductless 



