Physiology of Respiration, 



THE Object of respiration is to supply the oxygen necessary for the 

 oxidation processes that go on in the body, as well as to remove the 

 carbonic acid formed within the body. The most important organs 

 for this purpose are the lungs. There is an outer and an inner respira- 

 tion the former embraces the exchange of gases between the external 

 air and the blood-gases of the respiratory organs (lungs and skin) the 

 latter, the exchange of gases between the blood in the capillaries of the 

 systemic circulation and the tissues of the body. 



[The pulmonary apparatus consists of (1) an immense number of 

 small sacs the air-vesicles filled with air, and covered externally by 

 a very dense plexus of capillaries ; (2) air-passages the nose, pharynx, 

 larynx, trachea, and bronchi communicating with (1) ; (3) the thorax 

 with its muscles, acting like a pair of bellows, and moving the air within 

 the lungs.] 



106. Structure of the Air-Passages and Lungs. 



The lungs are compound tubular (racemose ?) glands which separate C0 2 from 

 the blood. Each lung is provided with an excretory duct (bronchus) which joins 

 the common respiratory passage of both lungs the trachea. 



Trachea. The trachea and extra-pulmonary bronchi are similar in structure. 

 The basis of the trachea consists of a number (16-20) of C-shaped incom- 

 plete cartilaginous hoops placed over each other. These rings consist of 

 hyaline cartilage, and are united to each other by means of tough fibrous tissue 

 containing much elastic tissue, the latter being arranged chiefly in a longitudinal 

 direction. The function of the cartilages is to keep the tube open under varying 

 conditions of pressure. Pieces of cartilage having a similar function occur in the 

 bronchi and their branches, but they are absent from the bronchioles, which are 

 less than 1 mm. in diameter. In the smaller bronchi the cartilages are fewer and 

 scattered more irregularly. [In a transverse section of a large intra-pulmonary 

 bronchus, two, three, or more pieces of cartilage, each invested by its peri- 

 chondrium, may be found.] At the points of bifurcation of the bronchi, the 

 cartilages assume the form of irregular plates embedded in the bronchial wall. 



An external fibrous layer of connective-tissue and elastic fibres covers the 

 trachea and the extra-pulmonary bronchi externally. Towards the oesophagus, the 

 elastic elements are more numerous, and there are also a few bundles of plain 

 muscular fibres arranged longitudinally. Within this layer there are bundles of 

 non-striped muscular fibres which pass transversely between the cartilages behind, 

 and also in the intervals between the cartilages. [These pale reddish fibres con- 



