Structure of the guinea-pig trachea at rest and in contraction.

The trachea of the guinea-pig measures about 47.5 mm in situ, and it shrinks to 38 mm when excised. It can be stretched to the in situ length with a load of 2-4 grams. The transverse area of its lumen measures about 4.5 mm2 in the cervical portion, whereas in the lowermost thoracic portion it measures 2.8 mm2, a difference of 37%. The lumen has an oval shape with the transverse diameter always exceeding the sagittal diameter. The separation between the ends of a cartilage in the dorsal region of the trachea is greater in the cervical than in the thoracic region. Elastic fibres are abundant in the tunica propria and run longitudinally; the collagen is in some areas arranged with a criss-cross pattern. The muscle is inserted on the concave aspect of each cartilage at some distance from the ends of the cartilage and it is therefore much longer than the gap between cartilage ends. Upon contraction in vitro (induced by carbachol) the muscle shortens by about 50%; there is a marked decrease of the transverse diameter of the trachea, and a certain decrease also of the sagittal diameter due to a straightening of the muscle and a change in shape and a movement of the mucosa. The cartilage ends are brought together and in the thoracic region they are bent and overlap extensively. The lumen of the trachea becomes circular and its area is reduced to 2.2 mm2 in the cervical portion and 1.7 mm2 in the thoracic portion.