TRACHEA. 779 



"border being prolonged downwards, and, at the same time, curved backwards, 

 at the point of bifurcation of the trachea. It terminates on each side in an im- 

 perfect ring, which incloses the commencement of the bronchi. The cartilage 

 above the last is somewhat broader than the rest at its centre. Two or more 

 of the cartilages often unite, partially or completely, and are sometimes bifur- 

 cated at their extremities. They are highly elastic, and seldom ossify, even in 

 advanced life. In the right bronchus, the cartilages vary in number from six 

 to eight; in the left, from nine to twelve. They are shorter and narrower than 

 those of the trachea. 



The Muscular Fibres are disposed in two layers, longitudinal and transverse. 

 The longitudinal fibres are the most external, and arise by minute tendons from 

 the termination of the tracheal cartilages, and from the fibrous membrane. 



The transverse fibres (Trachealis muscle, Todd and Bowman), the most inter- 

 nal, form a thin layer, which extends transversely between the ends of the car- 

 tilages, at the posterior part of the trachea. The muscular fibres are of the 

 unstriped variety. 



The Elastic Fibres are situated beneath the mucous membrane, inclosing the 

 entire cylinder of the trachea; they are most abundant at its posterior part, 

 where they are collected into longitudinal bundles. 



The Mucous Membrane lining the tube is covered with columnar ciliated 

 epithelium. It is continuous above with that of the larynx, and below with 

 that of the lungs. 



The Tracheal Glands are found in great abundance at the posterior part of the 

 trachea. They are small, flattened, ovoid bodies, placed between the fibrous 

 and muscular coats, each furnished with an excretory duct, which opens on the 

 surface of the mucous membrane. Some glands of smaller size are also found 

 at the sides of the trachea, between the layers of fibrous tissue connecting the 

 rings, and others immediately beneath the mucous coat. The secretion from 

 these glands serves to lubricate the inner surface of the trachea. 



Vessels and Nerves. The trachea is supplied with blood by the inferior thy- 

 roid arteries. The veins terminate in the thyroid venous plexus. The nerves 

 are derived from the pneumogastric and its recurrent branches, and from the 

 sympathetic. 



Surgical Anatomy. The air-passage may be opened in three different situations : through 

 the crico-thyroid membrane (laryngotomy), through the cricoid cartilage and upper ring of the 

 trachea (laryngo-tracheotomy), or through the trachea below the isthmus of the thyroid gland 

 (tracheotomy). The student should, therefore, carefully consider the relative anatomy of the 

 air-tube in each of these situations. 



Beneath the integument of the laryngo-tracheal region, on either side of the median line, are 

 the two anterior jugular veins. Their size and position vary; there is nearly always one, and 

 frequently two : at the lower part of the neck they diverge, passing beneath the Sterno-raastoid 

 muscles, and are frequently connected by a transverse communicating branch. These veins 

 should, if possible, always be avoided in any operation on the larynx or trachea. If cut through, 

 considerable hemorrhage occurs. 



Beneath the cervical fascia are the Sterno-hyoid and Sterno-thyroid muscles, the contiguous 

 edges of the former being near the median line; and beneath these muscles the following parts 

 are met with, from above downwards : the thyroid cartilage, the crico-thyroid membrane, the 

 cricoid cartilage, the trachea, and the isthmus of the thyroid gland. 



The crico-thyroid space is very superficial, and may be easily felt beneath the skin as a depres- 

 sion, about an inch below the pomum Adami; it is crossed transversely by a small artery, the 

 crico-thyroid, the division of which is seldom accompanied by any troublesome hemorrhage. 



The isthmus of the thyroid gland usually crosses the second and third rings of the trachea; 

 above it, is found a large transverse communicating branch between the superior thyroid veins, 

 and the isthmus is covered by a venous plexus, formed between the thyroid veins, of opposite 

 sides. On the sides of the thyroid gland, and below it, the veins converge to a single median 

 vessel, or to two trunks which descend along the median line of the front of the trachea, to open 

 into the innominate veins by valved orifices. In the infant, the thymus gland ascends a variable 

 distance along the front of the trachea : and the innominate artery crosses the tube obliquely at 

 the root of the neck, from left to right. The anterior thyroidea ima, when that vessel exists, 

 passes from below upwards along the front of the trachea. The upper part of the trachea lies 

 comparatively superficial ; but the lower part passes obliquely downwards and backwards, so as 

 to be deeply placed between the converging Sterno-mastoid muscles. In the child, the trachea 



