NEW VIEW OF THE MECHANISM OF RESPIRATION 277 



intervals. The external membrane, composed largely of inelastic and elastic fibres, is deficient posteriorly. In the 

 space devoid of cartilage two layers of unstriated involuntary muscular fibres occur — a thin, longitudinal, external 

 layer and a thick, deeper, transverse or circular layer. The longitudinal layer is confined to the trachea and large 

 bronchi ; the circular layer extending to the small bronchi, even to the air cells. 



On the mucous surface of the smaller bronchi cilia in large numbers are found. These, by their waving move- 

 ments, chiefly in an upward direction, keep the bronchi clear, by sweeping out extraneous substances. They also 

 assist in drawing air into the lungs by creating a counter current. They therefore perform a double function. A 

 certain amount of air always remains in the lungs, and is known as residual air. The air taken into and forced out 

 of the lungs during each inspiration and expiration is called tidal air. The essential part in the respiratory process 

 is the mixing of the carbonic acid of the blood contained within the capillaries of the air cells with the oxygen of 

 the inspired air contained within the air cells themselves. 



The mixing of the carbonic acid and oxygen is of the most intimate character, and takes place through the very 

 thin capillary walls of the air cells, which act as osmotic media. It also takes place by the intermingling and trans- 

 fusion of the gases themselves. In localising the respiratory function in man and in the lower animals it is necessary 

 to bear in mind that the aeration of the blood is not wholly confined to the lungs, but extends to every part of the 

 body where the capillaries and their contained blood, and other structures, are exposed to the action of the air. 



In animals the sldn, mucous membranes, and the tissues generally all take part in the respiratory process, so 

 that it is at once extensive and compUcated. It will be quite correct to say that the body breathes at every pore. 

 Similar remarks are to be made of plants, with this difference, that they give out oxygen and take in carbonic acid. 

 The breathing arrangements of plants and animals are the reverse of each other : the plant exhaling oxygen, which 

 is inhaled by the animal — the latter exhaling carbonic acid, which is inhaled by the plant. A balance is in this way 

 struck as between the gases consumed by the plant and animal respectively. This arrangement affords a striking 

 example of adaptation and of prevision and design. 



If an antero-posterior section be made of an adult cadaver in the frozen condition it will be seen that the lungs 

 with the heart and large blood-vessels completely occupy the chest (Plate Ixxvii., Fig. 6). The lungs with their 

 investments of pleura have not a cranny imoccupied. The lung pleura and the costal pleura are in contact, but not 

 adherent or united, so that the lungs are free to come and go and to enlarge and diminish with every enlargement 

 or diminution of the chest. The chest forms a vital, air-tight compartment, which is capable of increasing and 

 decreasing in all its diameters. When it increases, it draws out with it the lungs, which in that case are distended to 

 their utmost : when it diminishes, it compresses the lungs to a corresponding extent. The Hving chest therefore 

 exerts a vis a fronte and a vis a tergo function upon the lungs and upon the air ; the object being to bring fresh relays 

 of air and of oxygen to the lungs with a view to aerating the blood and to discharging a corresponding amount of 

 air containing an excess of carbonic acid added to it by the impure venous blood. It will be observed that the lungs 

 exteriorly are protected from atmospheric pressure by the walls of the chest and the diaphragm. They are also 

 protected interiorly because they are inflated with air at birth, which keeps them in a variable state of distension ever 

 after. While air is being constantly drawn into and ejected from the lungs, the lungs, as explained, are never wholly 

 devoid of it. The elastic properties of the lungs assist in keeping up the dehcate balance which enables them to 

 follow the ever- varying movements of the chest in respiration. 



The walls of the air cells of animals contain a large quantity of fine elastic fibres which ramify and anastomose 

 freely, and are not confined to one air cell ; an arrangement which gives rise to great distensibility and elasticity, pro- 

 perties of considerable importance in keeping the lungs always comfortably fifled with air. The presence of pale, 

 unstriated muscular fibres on the larger and smaller bronchi in such plenty suggests rhythmic movements in these 

 tubes. The smaller arteries similarly provided can undoubtedly open and close or partially open and close under 

 nervous guidance, as indicated by the excess of blood in the capillaries in blushing and the absence of blood in them 

 during fear, exposure to cold, &c. 



THE RESPIRATORY MOVEMENTS, ESPECIALLY IN MAN 



§ 56. New View of the Mechanism of Respiration— The Muscles of the Chest, Abdomen, and Diaphragm 

 all Involved. 



That blood-vessels have the power of opening and closing is proved by the rhythmic movements occurring in 

 the capillaries of the kidney, in the saphenous veins of the rabbit, in the vessels of the wing of the bat, and in the 

 large vessels at the base of the heart— the aorta and pulmonary artery, which are not supplied with muscular fibres, 

 excepted. 



