932 THE RESPIRATORY SYSTEM. 
it cuts across the eleventh rib and reaches the twelfth rib. The relation which it presents 
to the twelfth rib varies in accordance with the length of that rib. When the last rib is 
not abnormally short the pleura clothes its inner half, and the line of reflection falls 
below this portion of the rib so as to reach the spine midway between the head of the last 
rib and the transverse process of the first lumbar vertebra (Fig. 631). Here, therefore, 
the line of diaphragmatic reflection falls below the lower border of the thoracic wall, 
and this is a point of high practical importance. In operations upon the kidney the 
incision cannot be carried above the level of the transverse process of the first lumbar 
vertebra and the ligamentum arcuatum externum without the risk of wounding the pleura. 
On the right side the line of diaphragmatic pleural reflection differs from that on the 
left chiefly in front (Fig. 629, p. 929). Here it descends to a lower level. Thus it pro- 
ceeds outwards and downwards from the back of the ensiform cartilage along the. 
posterior aspect of the ascending part of the seventh costal cartilage, and it passes beneath 
the eighth costal arch, as a rule, at the same point as on the left side, viz. at the junction 
of its cartilaginous and bony parts. From this backwards to the spine the relations are 
so similar to those of the left side that a separate description is unnecessary. 
It is commonly stated that the left pleural sac reaches a lower level than the right. 
Tn certain cases there is no doubt that it does, but this condition is by no means the rule. 
In those cases where the two pleural sacs do not reach the same level at their lowest 
points, it is sometimes the right and sometimes the left pleura which oversteps the mark. 
As already stated, the lowest point to which the pleura descends is usually found, on 
both sides, in the mid-lateral line where the diaphragmatic reflection-line crosses the tenth 
rib or the tenth intercostal space. This point can be very readily ascertained on the 
surface by drawing a horizontal line round the trunk at the level of the lower part of the 
extremity of the spinous process of the first lumbar vertebra, and noting where it is 
intersected by the mid-lateral line. In the majority of cases the point of intersection will 
correspond with the lowest part of the pleural sac. Another horizontal line opposite the — 
spine of the last dorsal vertebra will give the level of the diaphragmatic pleural reflection — 
in the mammillary line.! 
Along the line of the diaphragmatic reflection a strong fascia passes from the 
lower uncovered part of the diaphragm, and from the costal cartilages to the surface y 
of the costal pleura, so as to hold it firmly in its place. It may be termed the phrenico- 
pleural fascia. 
a 
MEDIASTINAL OR INTERPLEURAL SPACE. ; 
The term mediastinal space is apphed to the interval between the mediastinal 
portions of the two pleural sacs. In front it is bounded by the sternum, and 
behind by the vertebral column. It is customary to subdivide this space in a 
purely arbitrary manner into four portions, termed respectively superior, anterior, 
middle, and posterior, according to the relations which they present to the 
pericardium. 
The superior mediastinum is the part of the general space which hes above the ~ 
level of the pericardium. Its boundaries are the folowing :—Jn front, the manu- 
brium sterni, to the posterior aspect of which are the attached lower ends of the — 
sterno-hyoid and sterno-hyoid muscles; behind, the bodies of the upper four dorsal 
vertebre ; below, an imaginary and oblique plane, which extends from the lower 
border of the manubrium sterni backwards and upwards to the lower border of the 
fourth dorsal vertebra ; laterally, the mediastinal pleura. | 
Within the superior mediastinal space are placed (1) the aortic arch and the 
three great arteries which spring from it; (2) the innominate veins and the upper 
part of the superior vena cava; (3) the trachea, gullet, and thoracic duct; (4) the 
phrenic, pneumogastric, left recurrent laryngeal and cardiac nerves; (5) the 
thymus gland. 
The middle mediastinum is the wide part of the space which contains the 
pericardium, and lies below the superior mediastinum. In addition to the peri- 
cardium and its contents the middle mediastinum contains the phrenic nerves and 
their accompanying vessels. * 
1 The above description represents the average results which have been obtained from the study of 
the pleura in a large number of subjects, eight of which were specially hardened by formalin or other 
re-agents for the purpose. For many of the dissections ] have to thank my former assistant Dr. H. 
St. J. Brooks, and for others I am indebted to my present assistant Dr. C. J. Patten. 
