222 RESPIRATION 



The object of all special respiratory arrangements being, in the first 

 instance, to facilitate the gaseous exchange between the surrounding 

 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 them- 

 selves sufficient to secure this renewal ; in other cases, artificial currents 

 are set up by cilia, or special bailing organs, like the soaphognathites 

 of the lobster. In all the higher animals, active movements 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 movements, 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 swallowing 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 innumerable 

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

 bronchiole breaks up into several wider passages, or infundibula, 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 between 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 infundi- 

 bula. 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 secre- 

 tion 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 

 apparently be derived from the vitiated blood of the right ventricle, 

 but is obtained directly from the aortic system by the bronchial arteries. 

 These are distributed with the bronchi, which they supply as well as 

 the connective tissue of the interlobular septa running through the 



