342 CLASS xvi. 



covered by a network of multangular cells, in each of which much 

 smaller cells are situated, which are surrounded by the capillaries 

 of the lungs and in which the chemical change from venous to 

 arterial blood takes place in respiration 1 . Through the apertures of 

 the bronchial tubes on the surface of the lungs the air passes into 

 large air-sacs which are situated partly in the thoracic partly in the 

 abdominal cavity, and which conduct air to the hollow bones. They 

 are supplied with blood vessels, but these are not branches of the 

 pulmonary arteries and moreover are not to be compared in number 

 and development with the blood-vessels of the lungs. So that if 

 to the air-sacs, as appendages of the lungs, any participation in the 

 respiratory function be ascribed, this cannot in any case be great. 

 But the air-sacs form reservoirs of air for breathing, and as they 

 receive air from the lungs, so they can return it to them again for 

 inspiration 2 . 



That birds have hollow bones filled with air had been observed 

 by earlier writers, by FABRICIUS AB AQUAPENDENTE, BORELLI, 

 and others; but still our celebrated countryman P. CAMPER was 

 the first (whilst JOHN HUNTER almost contemporaneously or 

 shortly after made the same discovery) who proved that these hol- 

 low bones communicate with the air-sacs, and therefore, mediately, 

 with the lungs 3 . The humerus, sternum and cranial bones are 



1 The finer structure of the lungs of birds, such as we have here described it, was 

 first made known by the investigations of A. EETZIUS, communicated to the Academy 

 of Sciences at Stockholm in 1831, and copied in FRORIEP'S Notizen, Bd. xxx. s. i 9, 

 figs. 9 n, 1838, 4to, pp. 56 58; see also LEREBOULLET Anat. comp. de I'Appareil 

 respiratoire, Strasbourg, 1838, 4to, pp. 56 58; ED. WEBER speaks of fine, closed 

 terminal tubes ; see Amtlicher Berkht uber die Versammlung dewtscher Naturforscher 

 in Braunschweig, 1845, 75 ; RAINEY Minute Anat. of the Lung of the Bird, Medico- 

 Chirurg. Transact, sec. Ser. xiv. pp. 47 58, PI. i ; WILLIAMS' article Respiration in 

 TODD'S Cyclop, of Anat. and Physiol , Suppl. 1855, pp. 276 278. 



3 See on these air-sacs CUVIER Lefons d'Anat. comp. iv. pp. 327 330, and the 

 second edition of DUVERNOY, vir. pp. 125 128, TIEDEMANN, 1.1. s. 601 618, OWEN, 

 1. 1. pp. 342, 343, and especially NATALIS GUILLOT Mem. sur I'Appardl de la Respira- 

 tion dans les Oiseaux, Ann. des Sc. natur. 30 Se*rie, v. 1846, Zoologie, pp. 25 87, 

 PL 3, 4. 



3 Compare on the hollow bones P. CAMPER in the Verh. van het Bataafsch Genoot- 

 schap te Rotterdam, i. bl. 235 244, with figures. (Also, with additions, in the French 

 edition of his works, Tom. ill. pp. 457 496.) His discovery was made in the year 

 i77 l In the same year that it was made public in the Transactions of the Dutch 

 Society, the observations of HUNTER appeared in the Philosoph. Transact. 1774; see 



