RESPIRATORY ORGANS. 19 



pose to state that respiration is necessary for the purification of 

 the venous blood, and the conversion of the chyle into blood, 

 and that the action of the air upon the blood is also the cause 

 of the internal heat, of which birds have a higher degree than 

 other animals. 



The external air is admitted to the lungs and to the cells dis- 

 persed over the body, and subsequently expelled from them, by 

 the alternate expansion and contraction of the thoracic and 

 abdominal cavities, effected by the action of the costal, abdo- 

 minal, and sternal muscles. Its passage takes place through a 

 tube, which commences behind the base of the tongue, opposite 

 to the internal aperture of the nares, extends along the fore-part 

 or side of the neck, enters the thorax between the clavicles, and 

 divides into two branches, of which one goes to each lung, and 

 there subdivides, as has been mentioned. This tube is named 

 the Windpipe, Trachea, or Aspera-arteria ; its two branches are 

 the Bronchi ; its commencement, or upper extremity, is the 

 Upper Larynx, or the Larynx ; and its lower extremity, or the 

 part at which it bifurcates, is the Loicer Larynx, or the Syrinx. 

 Besides acting as a pipe to the lungs, it is also the organ of 

 the voice, the air in passing through it causing its membranes 

 so to vibrate, and being so acted upon by the muscles attached 

 to it, as to give rise to the various cries and notes emitted by 

 birds, although the palate, the tongue, and the mandibles also 

 operate in modifying the sounds thus produced. 



At this stage of our lesson, we must have recourse to the 

 objects themselves. Having plucked one of our Blackbirds, 

 leaving only the feathers of the wings and tail, removed the 

 skin from the fore part of the neck, laid open the thorax on the 

 left side by dividing the pectoral muscles and ribs, and cut 

 away the anterior parietes of the abdomen, we observe the fol- 

 lowing parts. See Plate X, Fig. 1. The wings, feet, and tail, 

 require no particular notice at present. Behind the bill, a, are 

 seen a slender muscle, the stylo- hyoideus, b b, extending from 

 the lower jaw to the basal portion of the hyoid bone ; the 

 two branches of that bone, c c, enveloped by their muscles ; 

 and its slender medial or uro-hyal process, d. The oesophagus, 

 e, funnel-shaped at first, then tubular, and without dilatation or 



