FROZEN SECTIONS OF THE THORAX. 339 



These two figures may be compared with Fig. 101, 103, 107 and 109 in the present 

 work, and with Plates IV and V of Dwight'a " Frozen Sections of a Child " (B) 



§ 905. Level of the Sections. — So far as we can judge by comparison with dissections 

 and by the collation of these with the other sections of the same cat, the centrum of the 

 7th thoracic vertebra appears in Fig. 99, and that of the 6th in Fig. 100, the intervertebral 

 arthron being included in the thickness of the section. The section includes the hifurcation 

 of the trachea, which appears as a mesal tube in Fig. 100, into the two bronchi which 

 appear in Fig. 99 ; since plane surfaces only are shown, the ridge at the place of bifurca- 

 tion does not appear in Fig. 100. Between the oesophagus and the vertebra in Fig. 100 

 appear the transections of the MM. longus colli; but these terminate in the thickness of 

 the section, and in ¥ig. 99 the Vena azygus is seen to be joined by the first intercostal vein. 



§ 906. The Heart. — From the fact that the heart was injected with plaster, while the 

 lungs were not injected at all, the former occupies a disproportionally large space. The 

 natural obliquity of the organ also interferes with the ready appreciation of the relations 

 of the parts which appear in the two figures. 



In Fig. 99, the right and left correspond with those of the observer, while they are 

 reversed in Fig. 100. In the former, the vein presented includes a combination of the spe- 

 cial features which have been observed in the longitudinal section (Fig. 93) and the tran- 

 sections (Fig. 96, 97). The non-injection of the left auricle accounts for the relatively 

 larger size of the right in Fig. 99, and for the non-appearance of the left in Fig. 100. 



In Fig. 100, not only are the right and left parts reversed in position ^vith respect to 

 the observer, but the appearances are less readily comparable with what are shown in the 

 other figures of the heart. The right auricle is divided near the point of entrance of the 

 prcEcam, but the non-injected left auricle just escaped ; its cephalic end might fairly have 

 been placed in the vacant area just siuistrad of {dcxtrad of on the figure) the word A. pul- 

 moitalis. 



§ 907. Pleura (§ 806). — In both figures the lines representing the pleura are made dis- 

 proportionally heavy to facilitate their recognition. 



The pleura is seen to form a continuous line upon the ental aspect of the thoracic pari- 

 etes, to be reflected off at each side of the vertebral centrum upon the adjacent strtictures, 

 and then to follow the contour of the lung to the heart, where it forms the ectal lamina of 

 the pericardium. At the ventrimeson it is reflected again upon the parietes. Hence, like 

 the peritonetim (§ 725), each pleura is a closed sac with continuous walls, and the viscera 

 which appear to be within its cavity are really outside and in contact only with its ectal 

 surface. 



§ 908. Mediastinum. — The irregular space between the vertebra and the pericardium, 

 and bounded on the sides by the undulating line of pleura as it approaches the roots of the 

 lungs, is the mediastinum. Within it are the cemphagus and trachea, the aorta and other 

 vessels and nerves. 



§ 909. Septum mediastinale. — At the ventrimeson the right and left pleural reflec- 

 tions are in contact, and form an apparently single membrane between the two cavities, 

 which, as may be demonstrated very easily by experiment, are thus entirely disconnected. 

 In Fig. 99, the mediastinal septum so formed is very short, but in Fig. 100, the pericardium 

 is at some distance from the sternum and the septum is correspondingly extensive. 



In man (Quain, A, II, Fig. 163; Dwight, B, PI. IV), these two reflected layers of pleura 

 do not meet at the level of the heart, and the small space between them is called the 

 anterior mediastinum. For comparison with man, therefore, the single mediastinum of 

 the cat should be called dorsal or posterior. 



§ 910. Pericardium. — Like the pleura, the pericardium is represented upon the figures 

 by tmnaturally heavy lines. In Fig. 99, near the ventrimeson, it is seen to consist of two 



