THE THYROID GLAND. 979 



tissue of the parietes of the chest. This it may do without collecting in the pleural cavity ; 

 the 1 two layers of the pleura are so intimately in contact that the air passes straight through 

 from the wounded lung into the subcutaneous tissue. Emphysema constitutes, therefore, the 

 most important sign of injury to the lung in cases of fracture of the ribs. Pneumothorax, or 

 air in the pleural cavity, is much more likely to occur in injuries to the lung of the third variety ; 

 that is to say, from external wounds, from stabs, gunshot injuries, and such like, in which cases 

 air passes either from the wound of the lung or from an external wound into the cavity of the 

 pleura during the respiratory movements. In these cases there is generally no emphysema of 

 the subcutaneous tissue unless the external wound is small and valvular, so that the air drawn 

 into the wound during inspiration is then forced into the cellular tissue around during expiration 

 because it cannot escape from the external wound. Occasionally in wounds of the parietes of 

 the chest no air finds its way into the cavity of the pleura, because the lung at the time of the 

 accident protrudes through the wound and blocks the opening. This occurs where the wound 

 is large, and constitutes one form of hernia of the lung. Another form of hernia of the lung 

 occurs, though very rarely, after wounds of the chest wall, when the wound has healed and the 

 cicatrix subsequently yields from the pressure of the viscus behind. It forms a globular, elastic, 

 crepitating swelling, which enlarges during expiratory efforts, falls in during inspiration, and 

 disappears on holding the breath. 



THE THYROID GLAND. 



The thyroid gland is classified with the thymus, suprarenal capsules, and spleen, 

 under the head of ductless glands i. e., glands which do not possess an excretory 

 duct. From its situation in connection with the trachea and larynx, the thyroid 

 body is usually described with those organs, although it takes no part in the func- 

 tion of respiration. It is situated at the front and sides of the neck, and consists 

 of two lateral lobes connected across the middle line by a narrow transverse 

 portion, the isthmus. 



The weight of the gland is somewhat variable, hut is usually about one ounce. 

 It is somewhat heavier in the female, in whom it becomes enlarged during men- 

 struation and pregnancy. 



The lobes are conical in shape, the apex of each being directed upward and 

 outward as far as the junction of the middle with the lower third of the thyroid 

 cartilage ; the base looks downward, and is on a level with the fifth or sixth tracheal 

 ring. 



The external or superficial surface is convex, and covered by the skin, the 

 superficial and deep fascia, the Sterno-mastoid, the anterior belly of the Omo- 

 hyoid, the Sterno-hyoid and Sterno-thyroid muscles, and beneath the last muscle 

 by the pre-tracheal layer of the deep fascia, which forms a capsule for the gland. 



The deep or internal surface is moulded over the underlying structures, viz., 

 the thyroid and cricoid cartilages, the trachea, the inferior constrictor and posterior 

 part of the Crico-thyroid muscles, the oesophagus (particularly on the left side of 

 the neck), the superior and inferior thyroid arteries, and the recurrent laryngeal 

 nerves. 



Its anterior border is thin, and inclines obliquely from above downward and 

 inward toward the middle line of the neck, while the posterior border is thick and 

 overlaps the common carotid artery. Each lobe is about two inches in length, its 

 greatest width is about one inch and a quarter, and its thickness about three 

 quarters of an inch. 



The isthmus connects the lower third of the two lateral lobes; it measures 

 about half an inch in breadth and the same in depth, and usually covers the 

 second and third rings of the trachea. Its situation presents, however, many 

 variations, a point of importance in the operation of tracheotomy. In the middle 

 line of the neck it is covered by the skin and fascia, and close to the middle line, 

 on either side, by the Sterno-hyoid. Across its upper border runs a branch of 

 the superior thyroid artery ; at its lower border are the inferior thyroid veins. 

 Sometimes the isthmus is altogether wanting. 



A third lobe, of conical shape, called the pyramid, occasionally arises from the 

 upper part of the isthmus, or from the adjacent portion of either lobe, but most 

 commonly the left, and ascends as high as the hyoid bone. It is occasionally 

 quite detached, or divided into two or more parts, or altogether wanting. 



