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 carbon dioxide formed 

 within the body. The most important organs for this purpose are the lungs. 

 There is an outer and an inner respiration 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 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 16-20 C -shaped incomplete 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 perichondrium, may be found.] At the points where the bronchi subdivide, 

 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 pafe 

 reddish fibres constitute the trachealis muscle, and are attached to the inner surfaces of the 

 cartilages at a little distance from their free ends. The arrangement varies in different animals 

 thus, in the cat, dog, rabbit, and rat the muscular fibres are attached to the external surfaces 

 of the cartilages, while in the pig, sheep, and ox they are attached to their internal surfaces 

 {Stirling).'] Some muscular fibres are arranged longitudinally external to the transverse fibres. 

 The function of these muscular fibres is to prevent too great distension when there is great 

 pressure within the air-passages. 



The mucous membrane consists of a basis of very fine connective-tissue, containing much 

 adenoid tissue with numerous lymph-corpuscles. Numerous elastic fibres are arranged chiefly 

 in a longitudinal direction under the basement membrane. They are also abundant in the 

 deep layers of the posterior part of the membrane opposite the intervals between the cartilages. 

 A small quantity of loose sub-mucous connective-tissue containing the large blood-vessels, 

 glands, and lymphatics unites the mucous membrane to the perichondrium of the cartilages. 

 The epithelium consists of a layer of columnar ciliated cells with several layers of immature 



