68 GENERAL ANATOMICAL CHARACTERS 
column, the ribs, and the sternum. This is much arched, with the 
convexity towards the thorax, so that when its fibres contract and 
it is flattened the cavity of the thorax is increased, and when they 
are relaxed the cavity is diminished. 
LIungs.—The lungs are suspended freely in the thorax, one on 
each side of the heart, being attached only by the root, which 
consists of the bronchus or air-tube and pulmonary arteries and 
veins by which the blood is passed backwards and forwards between 
the heart and the lungs. The remaining part of the surface of 
each lung is covered by serous membrane, the “pleura”; and what- 
ever the state of distension or contraction of the chest-wall, is 
accurately in contact with it. Inspiration is effected by the con- 
traction of the diaphragm and by the intercostal and other muscles 
elevating or bringing forward the ribs, and thus throwing the 
sternum farther away from the vertebral column. As the surface 
of the lung must follow the chest-wall, the organ itself is expanded, 
and air rushes in through the trachea to fill all the minute cells in 
which the ultimate ramifications of the bronchi terminate. In 
ordinary expiration very little muscular power is expended, the 
elasticity of the lungs and surrounding parts being sufficient to 
cause a state of contraction and thus drive out at least a portion of 
the air contained in the cells, when the muscular stimulus is with- 
drawn. The lungs are sometimes simple externally, as in the 
Sirenia (where they are greatly elongated) and the Cetacea, but are 
more often divided by deep fissures into one or more lobes. The 
right lung is usually larger and more subdivided than the left. It 
often has a small distinct lobe behind, wanting on the left side, and 
hence called lobulus azygos. 
Air-sacs. —Most mammals have inconnection with the air passages 
certain diverticuli or pouches containing air, the use of which is 
not always easy to divine. The numerous air sinuses situated 
between the outer and inner tables of the bones of the head, 
represented in Man by the antrum of Highmore and the frontal and 
sphenoidal sinuses, and attaining their maximum of development 
in the Indian Elephant, are obviously for the mechanical purpose 
of allowing expansion of the osseous surface without increase of 
weight. They are connected with the nasal passages. The Eusta- 
chian tubes pass from the back of the pharynx to the cavity of the 
tympanum, into which and the mastoid cells they allow air to pass. 
In the Hquide there are large post-pharyngeal air-sacs in connection 
with them. The Dolphins have an exceedingly complicated system 
of air-sacs in connection with the nasal passages just within the 
nostrils, and the Tapirs, Rhinoceroses, and Horses have blind sacs 
in the same situation. In the males of some Seals (Cystophora and 
Macerorhinus) large pouches, which the animal can inflate with air 
and which are not developed in the young animal or the female, 
