486 THYROID GLAND LUNGS. 



they are covered in by a cellule-fibrous lamella, in which are lodged 

 the tracheal glands. These are small flattened ovoid bodies, situated 

 in great number between the fibrous and muscular layers of the 

 membranous portion of the trachea, and also between the two layers 

 of elastic fibrous tissue connecting the rings. They pour their 

 secretion upon the mucous membrane. 



Thyroid Gland. 



The thyroid gland is one of those organs which it is found ex- 

 tremely difficult to classify from the absence of any positive know- 

 ledge with regard to its function. It is situated upon the trachea, 

 and in an anatomical arrangement should therefore be considered 

 in this place, although bearing no part in the function of respiration. 



This gland consists of two lobes, which are placed one on each 

 side of the trachea, and are connected with each other by means of 

 an isthmus, which crosses its upper rings. There is considerable 

 variety in the situation and breadth of this isthmus ; which should 

 be recollected in the performance of operations upon the trachea. 

 In structure it appears to be composed of a dense cellular paren- 

 chyma, enclosing a great number of vessels. The gland is larger 

 in young subjects and in females, than in the adult and males. It 

 is the seat of an enlargement called bronchocele, goitre, or the Der- 

 byshire neck. 



A muscle is -occasionally found connected with its upper border 

 or with its isthmus ; and attached, superiorly, to the body of the os 

 hyoides, or to the thyroid cartilage. It was named by Soemmering 

 the " levator glandules thyroidea." 



Vessels and Nerves. It is abundantly supplied with blood by the 

 superior and inferior thyroid arteries. Sometimes an additional 

 artery is derived from the arteria innominata, and ascends upon the 

 front of the trachea to be distributed to the gland. The wounding 

 of this vessel, in tracheotomy, might be fatal to the patient. The 

 nerves are derived from the superior laryngeal and sympathetic. 



THE LUNGS. 



The lungs are two conical organs, situated one on each side of 

 the chest, embracing the heart, and separated from each other by a 

 membranous partition, the mediastinum. On the external or thoracic 

 side they are convex, and correspond with the form of the cavity 

 of the chest ; internally they are concave, to receive the convexity 

 of the heart. Superiorly they terminate in a tapering cone which 

 extends above the level of the first rib, and inferiorly they are broad 

 and concave, and rest upon the convex surface of the diaphragm. 

 Their posterior border is rounded and broad, the anterior sharp and 

 marked by one or two deep fissures, and the inferior which sur- 

 rounds the base is also sharp. 



The colour of the lungs is pinkish gray, mottled, and variously 



