CHAPTER III.

       THE CHEST AND ORGANS OF RESPIRATION.

The thorax, or chest, is a large chamber bounded laterally by
the ribs and their muscles, above by the spine, below by the breast-
bone, and behind by an obliquely placed musculo-tendinous partition
which separates it from the abdominal cavity, and known as the
diaphragm or midriff. It contains the heart and respiratory organs
and is deep and narrow. From researches conducted in recent
years by M. Alfred Giard * and others it has been found that the
pleural cavity, although present in the young elephant, is obliterated
in the adult. This is a condition entirely different from what
occurs in most mammals in which the lungs, though lying in contact
with, have no actual attachment to the walls. This peculiarity is
adapted to his mode of life, which seldom subjects him to respiratory
emergencies, and his organization which does not necessitate rapid
blood oxygenation.

In the lungs the blood cells are furnished with oxygen, which
being circulated by means of the heart distribute it to the tissues
which build up the system. Oxygen is necessary to the life of those
tissues. To effect this charging of the blood cells with oxygen
breathing is constantly going on, and consists of two acts : the first
during which air is conveyed through the respiratory passages to the
lungs to purify the blood (inspiratory act), and the second during
which air with impure products generated in the system is thrown
out (expiratory act). During inspiration the chest expands and on
expiration it contracts. In health this double act is performed on
an average 12 to 16 times a minute.

The respiratory organs.—The respiratory tract consists of—

     Nostrils.                                            Wind-pipe.
     Nasal chambers in the skull.                Lungs.

Nose.—The nasal apparatus consists of certain structures which
serve to form two passages, whose function, is to conduct air from
without into the air chambers in the bones of the skull, and also to

* Comptes rendus des seances de l'Academie des Sciences, CXLIV, page 306,
11th February 1907, by M. Alfred Giard.

Comptes rendus des stances de l'Academie des Sciences, CXLIV, page 471,
4th March 1907.

Comptes rendus des stances de l'Academie des Sciences, CXLIV, page 318,
17th June 1907.