276 



RESPIRATION. 



blood is poured into the pulmonary system at 

 the left side of the respiratory rete. The cur- 

 rent, therefore, entering the left auricle is not 

 pure arterial blood : it is alloyed by the venous 

 rivulet received from the bronchial system, 

 a reptilian characteristic traceable in human 

 organisation. 



By a third class of observers it is said, that 

 the capillaries of the pulmonary and those of 

 the bronchial system of vessels intimately 

 inosculate. The precise solution of this 

 question is difficult, in consequence of the 

 readiness with which an injection thrown into 

 one vessel will pass into another by extrava- 

 sation. Other anatomists suppose that the 

 three above-described modes of communica- 

 tion actually exist. It is certain that these 

 two systems do communicate, and that only a 

 part of the blood of the bronchial arteries re- 

 turns by the bronchial veins. More recently, a 

 new aspect has been given to this controversy 

 by the statements of Dr. Heale, to the effect 

 that the bronchial and the pulmonary systems 

 of vessels do not in any manner or degree 

 communicate. He maintains, on the evidence 

 afforded by his injections, that the vascular 

 web of the air-cells extends, and is prolonged 

 over the internal surfaces of the bronchial 

 tubes. Dr. Heale assigns to this extension of 

 the rete mirabile the power of prolonging the 

 aeration of the blood. This is impossible. 

 The bronchial tubes, the minutest, are inter- 

 nally lined by a dense ciliated epithelium. 

 Such epithelium does not exist on the true 

 capillary parts of the lungs of any vertebrated 

 animal. Where there is ciliated epithelium, a 

 universal principle of structure requires in the 

 higher vertebrated animals that the function of 

 breathing should be suppressed. This prin- 

 ciple, however, does not obtain in respiratory 

 organs of the invertebrata, and in the bron- 

 chial organs of lower vertebrata. 



Respiratory Organs of Birds. 



The lungs of birds are two in number, 

 symmetrically developed, flattened, and ir- 

 regularly triangular in figure. They are fixed, 

 by means of areolar tissue, to the ribs and 

 vertebral column, from the inequalities of 

 which they receive deep impressions. They 

 extend from the second dorsal vertebra as far 

 as the kidneys, and laterally to the junction 

 of the vertebral with the sternal ribs. In 

 their fixed position under the back and near 

 the centre of gravity, they contrast strikingly 

 with the lungs of mammals, which float 

 loosely in the thoracic chamber. In colour 

 they are blood-red, and in general texture 

 they are more fragile than the lungs of 

 mammals. They are not divided by deep 



" Par les arteres pulmonaires, Pinjection revenait 

 en entier par les veines correspondantes et jamais 

 par les arteres bronchiques." 



" Enfin, Pinjection pousse'e par leg veines pulmo- 

 naires remplissait tous les autres vaisseaux san- 

 guins du poumon, c'est-a-dire, Partere pulmonaire, 

 les arteres et les veines bronchiques." Rossignol, 

 Op. cit. p. 64. 



fissures into lobes, like the mammalian lung ; 

 lobuii, however, exist, although more length- 

 ened in form than those of the mammal 

 lung. In the former, as in the latter, a 



Fig. 225. 



A. Lobule of the lung of a Bird represented in ideal 



longitudinal section. ( Original. ) 

 a, a, a, primary bronchi maintaining a uni- 

 formity of diameter and terminating ccecally ; b, b, b, 

 second'ary bronchi, maintaining also a regularity 

 of diameter and opening into a dense cubic laby- 

 rinth of blood-vessels c, c. 



B. A small piece of the ultimate portion of the lung, 

 representing the arrangement of the ultimate re- 

 spiratory capillaries. 



lobule is a smaller lung. All its parts are 

 complete. The lobuii are embraced and 

 isolated by membranes of areolar elastic 

 tissue. A pleural investment embraces their 

 sternal aspects, and an aponeurosis, proceed- 

 ing from the diaphragmatic muscles below, 

 blends its fibres with those of this covering. 

 The trachea, after a course in the neck vary- 

 ing with the length of this part, at its entrance 

 into the lungs, divides into two primary 

 bronchi, one for each lung. At the place of 

 this bifurcation there exists, in most birds, 

 a complex mechanism of bones and cartilages, 

 moved by appropriate muscles, and consti- 

 tuting the true organ of voice. This part is 

 known as the inferior larynx.* The trachea 

 is composed of rings of cartilage which are 

 not deficient at the posterior third of the 

 circle, as in quadrupeds. The successive 

 rings are linked together into a cylindrical 

 form by means of a highly extensile and elastic 

 membrane. The whole cylinder is embraced 

 in a second concentric cylinder of muscular 

 fibre which belongs to the voluntary or striped 

 variety. In this particular it differs from the 

 trachea of mammals. In the latter, only the 

 deficient portion of the rings is composed of 



* See arts. LARYNX, VOICE, and art. AVE& 



