17 



from the inner wall of the thorax and the length of the thoracic cavity is 

 correspondingly increased. This increased thoracic space is immediately 

 occupied by the expanding lungs whose sharp borders then extend caudally 

 as far as the reflected costo-diaphragmatic pleura. In consequence of this 

 alternate contraction and relaxation of the diaphragm the lateral borders of 

 the lungs are continually moving backward and forward, the extent of such 

 movement varying from lo to 20 cm., PI. X. In expiration the caudal 

 border of the lung is approximately indicated by a line drawn from the 

 dorsal end of the twelfth rib to the costo-chondral junction of the sixth rib. 

 In inspiration the border of the lungs extends from 10 to 20 cm. caudal to 

 this Hne. 



The lungs. In connection with the lungs the following special points 

 may be noted : (a) The apex of the right lung, unlike that of the left, ex- 

 tends a little distance cephalad of the first rib, PI. II. (b) The extra bron- 

 chus to the apical lobe of the right lung arises from the trachea opposite the 

 third rib, Pis. Ill, IV, 17. (c) The bifurcation of the trachea into right 

 and left bronchi is opposite the fifth rib, PI. IV. {d) The cardiac notch of 

 the left lung is larger than that of the right so that a greater area of the 

 pericardium is left uncovered by the lung on the left side than on the right, 

 Pis. V, II. (e) On the left side the caudal portion of the lung overlaps the 

 spleen and on the right side it overlaps the liver, the diaphragm only inter- 

 vening, Pis. V, II. 



The pericardium and heart. 



The pericardium is covered by the middle or pericardiac portion of 

 the mediastinum and is crossed on either lateral side, near the base 

 of the heart, by the phrenic nerves. There are several small orifices 

 in the fibrous layer of the pericardium through which several of the 

 cardiac nerves pass to the walls of the heart. The lateral surfaces of 

 the pericarditun, in a strict sense, are related only to the mediastinal layers 

 of the pleura. Commonly, however, these surfaces of the pericardium are 

 described as though they were in direct contact with the lungs and -the 

 thoracic walls. On the left side the area of contact of the pericardium with 

 the thoracic wall is considerably larger than it is on the right side. In the 

 specimen, a large Holstein, from which these measurements were taken, this 

 area (cardiac notch of the lung) extended from the second rib to near the 

 caudal edge of the fourth rib. The area is approximately quadrilateral ; 

 its height, at the second rib, was 10 cm. and at the fourth rib 15 cm. dorsal 

 to the respective costo-chondral articulations. On the right side the cardiac 

 notch of the lung is smaller and consequently a smaller area of the peri- 



