MEDIASTINUM. 1089 



line of diaphragmatic reflection is carried downwards and laterally across the 

 extremities of the bony pprtions of the ninth and tenth ribs. As it crosses the 

 tenth rib, or, it may be, as it proceeds across the tenth intercostal space, the line of 

 pleural reflection reaches its lowest point, and it is important to observe that this 

 point lies in the mid-lateral line (i.e. in a vertical line drawn on the side of the 

 chest, midway between vertebral column and sternum). Thence, as it curves dorsally 

 towards the vertebral column, it passes slightly upwards. Thus 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 medial half, and the line of reflection falls below that portion of the rib, 

 so as to reach the vertebral column, midway between the capitulum of the last rib and the 

 transverse process of the first lumbar vertebra (Fig. 866). There, therefore, the line of 

 diaphragmatic reflection falls below the inferior border of the thoracic wall ; and this is 

 a point of practical importance, because in operations upon the kidney the incision cannot 

 be carried above the level of the transverse process of the first lumbar vertebra and the 

 lateral lumbo-costal arch without the risk of wounding the pleura. On the right side 

 the line of diaphragmatic pleural reflection differs from that on the left chiefly ventrally 

 (Fig. 864, p. 1086). There it passes at a lower level, and proceeds laterally and downwards 

 from the dorsum of the xiphoid process along the dorsal aspect of the seventh costal 

 cartilage, and then behind the eighth costal arch, as a rule at the same point as on the 

 left side, viz., the junction of its cartilaginous and bony parts. From that point to the 

 vertebral column 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. In 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 which the pleura attains 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. That point can be very readily ascertained on the 

 surface by drawing a horizontal line round the trunk at the level of the lowest 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 twelfth thoracic vertebra will give the level of the diaphragmatic pleural 

 reflection in the maniillary line. 1 



Along the line of the diaphragmatic reflection a strong fascia passes from the 

 uncovered part of the diaphragm, and from the costal cartilages to the surface of the 

 costal pleura, so as to hold it firmly in its place. It may be termed the phrenico- 

 pleural fascia. 



MEDIASTINUM. 



The term mediastinum is applied to the interval between the mediastinal 

 portions of the two pleural sacs. Ventrally it is bounded by the sternum, and 

 dorsally by the vertebral column. It is customary to subdivide this space in a 

 purely arbitrary manner into four portions, termed respectively the superior or 

 cranial, the ventral or anterior, the middle, and the dorsal or posterior part, 

 according to the relations which they present to the pericardium. 



The superior mediastinum is that part of the general area which lies above 

 the level of the pericardium. Its boundaries are as follows: Ventrally, the 

 manubrium sterni, with the attached sterno-hyoid and sterno-thyreoid muscles; 

 dorsally, the bodies of the first four thoracic vertebrae ; below, an imaginary and 

 oblique plane, which extends from the inferior border of the manubrium sterni to 

 the inferior border of the fourth thoracic vertebra ; laterally, the mediastinal pleura. 



Within the superior mediastinum are placed (1) the aortic arch and the three 



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 I have to thank my former assistant, Dr. H. 

 it. J. Brooks, and for others I am indebted to Professor C. J. Patten of Sheffield. D. J. C. 



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