264 A MANUAL OF ANA TOMY. 



varies, of course, with the descent of the lungs during 

 inspiration. 



(13) The Line Indicating the Level of the Pleurce, 

 This is lower than that of the lungs. It is represented 

 by a line starting at the sternum at the articulation of the 

 seventh cartilage, and passes obliquely around the chest to 

 the spine of the eleventh dorsal vertebrae. This line crosses 

 the lower border of the ninth rib in the axillary line, and 

 behind is as low as the upper border of the twelfth rib. 



(14) The trachea bifurcates on a level with the articu- 

 lation between the first and second pieces of the sternum. 



(15) The internal mammary artery runs vertically 

 downward behind the costal cartilages and intercostal mus- 

 cles one-half inch external to the sternal border as low as 

 the sixth cartilage, where it divides into its two terminal 

 branches, the superior epigastric and musculophrenic. 



DISSECTION. 



Incisions. I. In the median line from the top, to the bottom of the sternum. 



2. From the beginning of I , out along the clavicle to the point of the 

 shoulder (same incision as 2, page 69). 



3. From the lower end of I, transversely around the body, as far as the 

 posterior axillary line (same as 2, page 435). 



4. From the centre of the clavicle down the front of the arm to the elbow. 

 Remove the integument from the chest and upper part of the arm. 



The Superficial Fascia. 



This sheet of subcutaneous tissue covers the entire area. 

 It is continuous with the similar fascia of the abdomen, 

 neck, back, shoulder, and arm. Its thickness depends 

 upon the amount of adipose tissue present in its meshes. 

 Within the fascia are found the subcutaneous vessels, nerves, 

 and the mammary gland. 



Superficial Vessels and Nerves and Origin of Platysma. 

 At the upper part of the chest the origin of the platysma 



