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. 



Lungs. — 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 in connection 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 Equiclce 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 

 Macrorhinus) large pouches, which the animal can inflate with air, 

 and which are not developed in the young animal or the female, 



