502 PUNCTURE OF THE THORAX. 



on the side. The operation is not now performed as low down as 

 formerly. In all animals the opening should be made either slightly 

 above or slightly below the subcutaneous thoracic vein, keeping 

 clear of the large pectoral muscle, which lies still lower. The upper 

 position deserves preference. Where both pleural sacs contain 

 fluid, the operation should be performed on the right side, to prevent 

 injuring the heart. Sometimes, however, the foramina of the posterior 

 mediastinum are blocked, and it becomes necessary to puncture 

 both sides of the chest. The vertical boundary for punctures lies 

 in the horse between the 5th and 8th ribs ; in oxen, between the 

 6th and 8th ; in swine, between the 7th and 9th ; and in carnivora, 

 between the 5th and 8th. In actual practice the ribs are often dis- 

 regarded, and in the horse the puncture is made a span behind the 

 point of the elbow. The instrument should be inserted close to the 

 anterior edge of the posterior rib of the selected intercostal space. 

 (For relations of the ribs to thoracic contents, see Figs. 410 and 411.) 

 The hair is first shaved from the site of operation, the skin washed 

 with soap and rinsed with ether, and finally saturated with a dis- 

 infecting fluid. Similar care must be taken in disinfecting the trocar. 

 In practice this is most easily and safely effected by boiling the 

 instrument in water, or laying it in a 3 to 5 per cent, solution of 

 carbolic acid. To facilitate the entrance of the instrument an incision 

 may first be made through the skin with a bistoury or lancet. The 

 trocar, grasped with the handle in the hollow of the right hand, is 

 supported with the thumb and fingers, while the index finger, more 

 or less extended, is fixed on the cannula at the point to which the 

 trocar is to enter. This distance varies according to the thickness 

 of the thoracic wall, and in the larger animals varies from 1£ to 2 \ 

 inches. The trocar is introduced perpendicularly to the chest-wall 

 — until the index finger meets the skin. To prevent entrance of 

 air the skin should be pulled forward or backward before the puncture 

 is made. The stilette is then withdrawn with the right hand, whilst 

 the left supports the cannula, and the left index finger is held ready 

 to close the opening against entrance of air, which may occur when 

 the flow of fluid ceases, when inspiration is specially deep, or when 

 coughing ensues. If the discharge of fluid is prematurely checked 

 by clots of lymph or blood, or by apposition of the lung, a few slight 

 lateral movements should be made with the cannula, and the stilette 

 or a carefully-disinfected probe passed through it. 



Schuh has devised a trocar, carrying at its outer end a reservoir 

 (trough or bath), so that the opening of the cannula is always 

 immersed in fluid ; but this arragement is not reliable, and is little 



