THYROID GLAND. 549 



The Fibro-cartilaginous rings are from fifteen to twenty in number, 

 and extend for two-thirds around the cylinder of the trachea. They 

 are deficient at the posterior part, where the tube is completed by 

 fibrous membrane. The last ring has usually a triangular form in 

 front. The rings are connected to each other by a membrane of yellow 

 elastic fibrous tissue, which 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 ultimate terminations. 



The Muscular fibres form a thin layer, extending transversely be- 

 tween the extremities of the cartilages. On the posterior surface they 

 are covered in by a cellulo-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 membra- 

 nous 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 

 arrangement may therefore be considered in this place, although bear- 

 ing 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 ap- 

 pears to be composed of a dense cellular parenchyma, 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 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 glandulae thyroidece ;" fig. 163, p. 543. 



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. 



