1122 THE ORGANS OF VOICE AND RESPIRATION. 



process of the seventh cervical vertebra, down either side of the spine, corresponding to the 

 costo-vertebral joints as low as the spinous process of the tenth dorsal vertebra. The trachea 

 bifurcates opposite the spinous process of the fourth dorsal vertebra, and from this point the two 

 bronchi are directed outward, the right one almost horizontally, the left with a considerable 

 inclination downward. 



The position of the great fissure in the right lung may be indicated by a line drawn from 

 the fourth dorsal vertebra round the side of the chest to the anterior margin of the lung opposite 

 the seventh rib, and the smaller or secondary fissure by a line drawn from the preceding where 

 it bisects the mid-axillary line to the junction of the fourth costal cartilage to the sternum. The 

 great fissure in the left lung is a little higher, extending from the third dorsal vertebra round the 

 side of the chest to reach the anterior margin of the lung opposite the sixth costal cartilage. 



Surgical Anatomy. The lungs may be wounded or torn in three ways: (1) By compres- 

 sion of the chest, without any injury to the ribs. (2) By a fractured rib penetrating the lung. 

 (3) By stabs, gunshot wounds, etc. 



The first form is very rare, and usually occurs in young children, and affects the root of the 

 lung i. e. the most fixed part and thus, implicating the great vessels, is frequently fatal. Its 

 exact mode of causation is difficult to interpret. The probable explanation is that immediately 

 before the compression is applied a deep inspiration is taken and the lungs are fully inflated ; 

 owing then to spasm of the glottis at the moment of compression, the air is unable to escape 

 from the lung, which is not able to recede, and consequently gives way. 



In the second variety both the pleura costalis and pulmonalis must necessarily be injured, 

 arid consequently the air taken into the wounded air-cells may find its way through these wounds 

 into the cellular tissue of the parietes of the chest. This it may do without collecting in the 

 pleural cavity ; the 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. Pneu- 

 mothorax, or air in the pleural cavity, is much more likely to occur in injuries to the lung of 

 the third variety, in which cases air passes either from the wound of the lung or from 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 val- 

 vular, 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. Occasion- 

 ally 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 open- 

 ing. 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 bears much resemblance in structure to other glandular 

 organs, and is classified, together with the thymus, suprarenal capsules, and 



FIG. 712. Two lobules from the thyroid of an infant, a. Small glandular vesicles with their cells, />. The 

 same, with incipient colloid metamorphosis, more strongly marked at c. d. Coarse lymph-canals, e. Fine rad- 

 icles of the same. f. An efferent vessel of considerable size. 



spleen, under the head of ductless glands, since when fully developed it has no 

 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 



