blush depending upon the intensity of the emotion.

The pallor produced by fright and by extreme anxiety, is purely the
result of a local modification of the circulation, brought about by an
over-stimulation of the nerves which supply the small arteries, causing
them to contract, and to thus cut off more or less completely the supply
of blood.

* * * * *




CHAPTER VIII.

PHYSIOLOGICAL ANATOMY.

THE ORGANS OF RESPIRATION.


THE ORGANS OF RESPIRATION are the Trachea, or windpipe, the Bronchia,
formed by the subdivision of the trachea, and the Lungs, with their
air-cells. The _Trachea_ is a vertical tube situated between the lungs
below, and a short quadrangular cavity above, called the _larynx_, which
is part of the windpipe, and used for the purpose of modulating the
voice in speaking or singing. In the adult, the trachea, in its
unextended state, is from four and one-half to five inches in length,
about one inch in diameter, and, like the larynx, is more fully
developed in the male than in the female. It is a fibro-cartilaginous
structure, and is composed of flattened rings, or segments of circles.