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cartilages, cartilage and thyroid

TRACHEA, tra-ke'a or tra'ke-a, or WIND PIPE, the principal air-passage of the body; a tube extending from the larynx to a point opposite the' third dorsal vertebra, where the tube divides into two chief divisions or bronchi, one of which supplies each lung with the air necessary for respiration or breathing. The trachea is of cylindrical form and is both mem branous and cartilaginous in its structure. Its length is about four and one-half inches and its diameter from three-fourths inch to one inch; that of the male being larger than that of the female. The front or anterior surface of the organ is convex and is covered in the necic and chest by various structures, including the isthmus of the thyroid gland, the inferior thyroid veins, the sternohyoid and sternothyroid muscles, the first part of the sternum, the arch of the aorta, etc. The trachea rests on the gullet or cesophagus, while in the chest it is situated between the pleurx or membranes lining the thorax and has the pneumogastric nerve on each side. The trachea is composed of rings

or zones of a gristly' or cartilaginous nature, known as the cartilages of the trachea. It rests on the gullet or cesophagus, while each cartilage forms an imperfect ring, being unclosed behind and having the gristly edges merely joined by fibrous membrane. The cartilages are sepa rated from each other and also connected to gether by narrow bands of fibrous tissue. The first cartilage of the trachea is broader than the others and may be divided at one extremity, while the last cartilage is thick in the middle and curved backward at the point where the trachea divides into the two bronchi. Some times two of the c.artilages may unite. The muscular fibres of the trachea exist in longi tudinal and transverse layers and are composed of unstriped or non-striated fibres. (See Muscu