THE PLEURAE. 969 



Tracheotomy may be performed either above or below the isthmus of the thyroid body or 

 this structure may be divided and the trachea opened behind it. 



The isthmus of the thyroid gland usually crosses the second and third rings of the trachea ; 

 along its upper border is frequently to be found a large transverse communicating branch between 

 the superior thyroid veins ; and the isthmus itself is covered by a venous plexus formed between 

 the thyroid veins of the opposite sides. Theoretically, therefore, it is advisable to avoid dividing 

 this structure in opening the trachea. 



Above the isthmus the trachea is comparatively superficial, being covered by the skin, super- 

 ficial fascia, deep fascia, Sterno-hyoid and Sterno-thyroid muscles, and a second layer of the 

 deep fascia, which, attached above to the lower border of the hyoid bone, descends beneath the 

 muscles to the thyroid body, where it divides into two layers and encloses the isthmus. 



Below the isthmus the trachea lies much more deeply, and is covered by the Sterno-hyoid 

 and the Sterno-thyroid muscles and a quantity of loose areolar tissue in which is a plexus of 

 veins, some of them of large size ; they converge to two trunks, the inferior thyroid veins, which 

 descend on either side of the median line on the front of the trachea and open into the innomi- 

 nate veins. In the infant the thymus gland ascends a variable distance along the front of the 

 trachea, and opposite the episternal notch the windpipe is crossed by the left innominate vein. 

 Occasionally also, in young subjects, the innominate artery crosses the tube obliquely above the 

 level of the sternum. The thyroidea ima artery, when that vessel exists, passes from below up- 

 ward along the front of the trachea. 



From these observations it must be evident that the trachea can be more readily opened 

 above than below the isthmus of the thyroid body. 



Tracheotomy above the isthmus is performed thus : the patient should, if possible, be laid 

 on his back on a table in a good light. A pillow is to be placed under the shoulders and the 

 head thrown back and steadied by an assistant. The surgeon standing on the right side of his 

 patient makes an incision from an inch and a half to two inches in length in the median line of 

 the neck from the top of the cricoid cartilage. The incision must be made exactly in the middle 

 line, so as to avoid the anterior jugular veins, and after the superficial structures have been 

 divided the interval between the Sterno-hyoid muscles must be found, the raphe divided, and 

 the muscles drawn apart. The lower border of the cricoid cartilage must now be felt for, and 

 the upper part of the trachea exposed from this point downward in the middle line. Bose has 

 recommended that the layer of fascia in front of the trachea should be divided transversely at 

 the level of the lower border of the cricoid cartilage, and, having been seized with a pair of 

 forceps pressed downward with the handle of the scalpel. By this means the isthmus of the 

 thyroid gland is depressed, and is saved from all danger of being wounded, and the trachea 

 cleanly exposed. The trachea is now transfixed with a sharp hook and drawn forward in order 

 to steady it, and is then opened by inserting the knife into it and dividing the two or three 

 upper rings from below upward. If the trachea is to be opened below the isthmus, the incision 

 must be made from a little below the cricoid cartilage to the top of the sternum. 



In the child the trachea is smaller, more deeply placed, and more movable than in the adult. 

 In fat or short-necked people, or in those in whom the muscles of the neck are prominently 

 developed, the trachea is more deeply placed than in the opposite conditions. 



A portion of the larynx or the whole of it has been removed for malignant disease, laryng- 

 ectomy. The results which have been obtained from the removal of the whole of it have not 

 been very satisfactory, and the cases in which the operation is justifiable are very few. It may 

 be removed by a median incision through the soft parts, freeing the cartilage from the muscles 

 and other structures in front, separating the larynx from the trachea below, and dissecting off 

 the deeper structure from below upward. 



THE PLEURA. 



Each lung is invested, upon its external surface, by an exceedingly delicate 

 serous membrane, the pleura, which encloses the organ as far as its root, and is 

 then reflected upon the inner surface of the thorax. The portion of the serous 

 membrane investing the surface of the lung and dipping into the fissures between its 

 lobes, is called the pleura pulmonalis (visceral layer of pleura), while that which 

 lines the inner surface of the chest is called the pleura costalis (parietal layer of 

 pleura). The space between these two layers is carled the cavity of the pleura, 

 but it must be borne in mind that in the healthy condition the two layers are in 

 contact, and there is no real cavity until the lung becomes collapsed and a separa- 

 tion of it from the wall of the chest takes place. Each pleura is therefore a shut 

 sac, one occupying the right, the other the left half of the thorax, and they are 

 perfectly separate from each other. The two pleurae do not meet in the middle line 

 of the chest, excepting anteriorly opposite the second and third pieces of the ster- 

 num a space being left between them, which contains all the viscera of the thorax 

 excepting the lungs : this is the mediastinum. 



Reflections of the Pleura (Fig. 535). Commencing at the sternum, the pleura 

 passes outward, lines the costal cartilages, the inner surface of the ribs and 



