INSPIRATION 



95 



vessels are larger than the capillaries in other situations, and the plexus 

 is so close that the spaces between them are narrower than the vessels 

 themselves. When distended, the bloodvessels form the greatest part 

 of the walls of the cells. 



Lining the air-cells, are thin cells of flattened epithelium, 25^0 to 

 ToVo .f an inch ( I0 to I2 -5 /*) in diameter, which are applied directly 

 to the walls of the bloodvessels. The epithelium here does not seem to 

 be regularly desquamated as in other situations. Examination of in- 

 jected specimens shows that the bloodvessels are so situated between 

 the cells that the blood in the greater part of their circumference is 

 exposed to the air (see Plate II, Fig. 5). 



The entire mass of venous blood is distributed in the lungs by the 

 pulmonary artery. Arterial blood is conveyed to the lungs by the bron- 

 chial arteries, which ramify and subdivide on the bronchial tubes and 

 follow their course into the lungs, for the nourishment of these parts. 

 It is possible that the tissue of the lungs may receive some nourishment 

 from the blood of the pulmonary artery ; but as this vessel does not 

 send branches to the bronchial tubes, the bronchial arteries supply the 

 matters for their nutrition and for the secretion by the mucous glands 

 (see Plate II, Fig. 4). 



MOVEMENTS OF RESPIRATION 



In man and in the warm-blooded animals generally, inspiration takes 

 place as a consequence of enlargement of the thoracic cavity and the 

 entrance of air through the respiratory passages, corresponding with the 

 increased capacity of the lungs. In the mammalia, the chest is en- 

 larged by the action of muscles ; and in ordinary respiration, inspiration 

 is an active process, while ordinary expiration is mainly passive. 



The walls of the thorax are formed by the dorsal vertebrae and ribs 

 posteriorly, by the upper ten ribs laterally, and by the sternum and 

 costal cartilages anteriorly. The direction of the ribs, their mode of 

 connection with the sternum by the costal cartilages, and their articula- 

 tion with the vertebral column are such that by their movements, the 

 antero-posterior and transverse diameters of the chest may be consider- 

 ably modified. 



Inspiration. The ribs are somewhat twisted upon themselves and 

 have a general direction forward and downward. The first rib is nearly 

 horizontal, but the obliquity of the ribs progressively increases from the 

 upper to the lower part of the chest. They are articulated with the 

 bodies of the vertebrae so as to allow of considerable motion. The 

 upper seven ribs are attached by the costal cartilages to the sternum, 



