RESPIRA TION 207 



The object of all special respiratory arrangements being, in the 

 first instance, to facilitate the gaseous exchange between the sur- 

 rounding medium (air or water) and the blood, a prime necessity of 

 a respiratory organ, be it skin, gill, trachea, or lung, is a free supply 

 of blood, in vessels so fine and thin that diffusion readily takes place 

 into them and out of them. But a free supply of blood would be of 

 no avail if the medium to which the blood gave up its carbon dioxide 

 and from which it drew its oxygen was not being constantly and 

 sufficiently renewed. 



Sometimes the natural currents of the water or the air are of 

 themselves sufficient to secure this renewal ; in other cases, artificial 

 currents are set up by cilia, or special bailing organs, like the scapho- 

 gnathites of the lobster. In all the higher animals, active move- 

 ments, by which air or water is brought into contact with the 

 respiratory surfaces, are necessary ; and it is possible that such 

 movements take place even in the tracheae of insects and other 

 air-breathing arthropoda. Fishes, by rhythmical swallowing move- 

 ments, take in water through the mouth and pass it over the gills 

 and out by the gill-slits, while the frog distends its lungs by swal- 

 lowing air. 



Physiological Anatomy of the Respiratory Apparatus. In man 

 the respiratory apparatus consists of a tube (the trachea) widened at 

 its upper part into the larynx, which contains the special mechanism 

 of voice, and communicates through the nose or mouth with the 

 external air. Below, the trachea divides dendritically into innumer- 

 able branches, the ultimate divisions of which are called bronchioles. 

 Each bronchiole breaks up into several wider passages, or inf undibula, 

 the walls of which are everywhere pitted with recesses or alcoves, 

 called alveoli. The infundibula constitute the essential distensible 

 elements of the lung, by the alternate stretching and relaxation of 

 which the respiratory changes in the volume of the organ are mainly 

 brought about. The trachea and larger bronchi are strengthened by 

 hyaline cartilage in the form of incomplete rings, connected behind 

 by non-striped muscular fibres, which also exist in the intervals be- 

 tween the rings. The middle-sized bronchi within the lungs have the 

 cartilage in the form of detached pieces in the outer portion of the 

 wall, while nearer the lumen lies a complete ring of non-striped muscle. 



In the bronchioles, no cartilage is present, but the circularly- 

 arranged muscular fibres still persist, and also form a thin layer in 

 the infundibula. In the air-cells, or alveoli, however, there are no 

 muscular fibres. Their walls consist essentially of a network of 

 elastic fibres, continuous with a similar layer in the infundibula and 

 bronchioles, and covered on the side next the lumen by a single 

 layer of large, clear epithelial scales, with here and there a few 

 smaller and more granular polyhedral cells. 



From the larynx to the bronchioles the mucous membrane is 

 ciliated on its free surface, the cilia lashing upwards so as to move 

 the secretion towards the larynx and mouth. In the infundibula the 

 ciliated epithelium begins to disappear, and is absent from the alveoli. 

 Part of the nasal cavity and the upper part of the pharynx are also 

 lined with ciliated epithelium. Mucous glands are present in 

 abundance in the upper portions of the respiratory passages, but 

 disappear in the smaller bronchi. 



Blood-supply of the Lungs. The quantity of blood traversing the 

 lungs bears no proportion to the amount required for their actual 

 nourishment. Small, however, as this latter quantity is, it cannot 



