492 THE LUNGS. 



are connected to each other by a membrane of yellow elastic fibrous tissue^ 

 \vhich in the space between the extremities of the cartilages, posteriorly, 

 forms a distinct layer. 



The Longitudinal elastic fibres are situated immediately beneath the 

 mucous membrane on the posterior part of the trachea, and enclose the 

 entire cylinder of the bronchial tubes to their terminations. 



The Muscular fibres form a thin layer, extending transversely between 

 the extremities of the cartilages. On the posterior surface they are covered 

 in by an areolo-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 or body is one of those organs which it is difficult to 

 classify from the absence of any positive knowledge with regard to its 

 function. It is situated upon the trachea, and in an anatomical arrange- 

 ment may 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 situa- 

 tion and breadth of the isthmus, which should be recollected in the per- 

 formance of operations upon the trachea. In structure it is composed, 

 according to Mr. Simon,* of a dense aggregation of minute and inde- 

 pendent membranous cavities enclosed by a plexus of capillary ves- 

 sels, and connected together by areolo-fibrous tissue. The cavities are 

 filled with fluid, in which are found cyto-blasts and cells ; the latter mea- 

 suring T^jff of an inch in diameter. In young animals the cyto-blasts lie 

 in contact with the internal wall of the cavities, and constitute a kind of 

 tesselated epithelium. 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 Derbyshire 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 

 thyroidc? 1 fig. 214, p. 487. 



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

 rior 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 the latter ves- 

 sel, 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 that organ 



* The Comparative Anatomy of the Thyroid Gland. Philosophical Transactions, 

 '844 



