246 ESSENTIALS OF PHYSIOLOGY. 



and elastic tissue ; the network of capillaries is thus common to the 

 two adjacent air cells, and the blood in the capillaries is separated from 

 the air in the alveoli merely by two thin layers of epithelium. In 

 birds, even the alveolar epithelium appears to be absent, the blood and 

 air being separated solely by the capillary wall. 



The branches of the pulmonary artery accompany the bronchi, and 

 open into the alveolar capillary network, from which blood is carried 

 back to the left auricle by the pulmonary veins. Oxygenated blood is 

 supplied to the bronchi by the bronchial arteries. 



The lungs nearly fill the thoracic cavity, the space between them 

 being occupied by the heart, great vessels, and other structures. Each 

 lung is covered by a thin membrane, consisting of a superficial layer 

 of flattened epithelium resting on connective tissue; the membrane 

 is known as the pleura, and is reflected at the root of the lung on 

 to the chest wall. Each pleura thus forms a closed sac, the walls 

 of which are normally in apposition ; their inner surfaces are moistened 

 by a small amount of fluid, resembling lymph, and glide over one 

 another with every movement of the chest wall and lung. 



THE RESPIRATORY MOVEMENTS. 



The thorax is a completely closed box which alters in shape and 

 size with each respiratory movement ; with inspiration it becomes larger 

 in all its diameters, vertical, antero-posterior, and transverse, returning 

 to its former size during expiration. This increase in size is brought 

 about partly by the upward movement of the ribs, partly by the descent 

 of the diaphragm. 



The diaphragm consists of a muscular sheet with a tendinous 

 central portion. In the position of rest it forms a dome projecting 

 towards the thoracic cavity, and when it contracts the summit of the 

 dome, namely the tendinous portion, is drawn downwards from 1 to 2 cm., 

 thus increasing the vertical diameter of the chest. The extent to which 

 the central tendon can be drawn downwards is limited by the resistance 

 of the abdominal viscera, and when the limit is reached, the direction 

 of the pull of the costo-sternal muscle fibres of the diaphragm is 

 reversed so that the lower end of the sternum and the movable ribs 

 are raised. The spinal fibres of the diaphragm, the lower attachment 

 of which is a fixed point, still exert a downward pull upon the central 

 tendon. 



The Ribs. At the beginning of inspiration the first pair of ribs 

 and the manubrium sterni are fixed by the resistance of the cervical 

 structures, and the second to the fifth pairs of ribs are drawn upwards 

 by the contraction of the external intercostal muscles and the serratus 



