_Nervous Mechanism._--Nervously, respiration is controlled in three ways: 1. by the phrenic nerve to the diaphragm; 2. by some fibers of the vagus or pneumogastric, and 3. by the respiratory center in the bulbous portion of the spinal cord. Injury to the respiratory center means the ceasing of respiration and death. Stimulation of the respiratory center seems to depend upon the character of the blood. If it is well oxygenized, the breathing is slow and quiet; if there is a lack of oxygen, dyspnœa results. Probably certain chemical substances in the blood, which are ordinarily rapidly burned up by the oxygen but which accumulate in its absence, serve to stimulate the respiratory center, thus adjusting the effort to get oxygen to the need of it. Respiration may be stopped by stimulating the mucous membrane of the nose, as with strong ammonia. _Variations._--Certain variations from the ordinary respiration might be mentioned here. A deep inspiration followed by a long expiration is known as a _sigh_ and a very deep inspiration through the mouth only as a _yawn_. _Hiccough_ results from a sudden inspiratory contraction of the diaphragm during which the glottis is suddenly closed. In _sobbing_ the inspirations are short and rapid with a prompt closing of the glottis between. Both _coughing_ and _sneezing_ consist of a deep inspiration followed by complete closure of the glottis and then its sudden opening and the forcible expulsion of air. Coughing, however, is generally caused by an irritation or obstruction of the larynx or trachea and the air is expelled through the mouth, while sneezing is caused by irritation of the nasal passages and the air is driven out through the nose. _Laughing_ and _crying_ also resemble one another in that each is an inspiration followed by a series of short, spasmodic expirations, during which the glottis is open and the vocal cords in characteristic vibration. They differ, however, in rhythm and in the facial expression that accompanies them. CHAPTER X. THE ABDOMEN AND THE ORGANS OF DIGESTION AND EXCRETION. =The Abdominal Cavity.=--Below the diaphragm and separated from the lowest cavity of the trunk, the pelvis, only by an invisible plane drawn through the brim of the true pelvis, is the abdominal cavity, which may be said in a general way to contain the organs of digestion and the kidneys. It is protected behind by the vertebræ and anteriorly by the lower ribs above and below by muscular walls, which make possible the complete bending of the body. These muscles are for the most part large and very strong and the greater number are inserted, in part at least, into a median tendinous line, the _linea alba_, which passes from the ensiform cartilage of the sternum above to the symphysis pubis below.