250 NATURAL HISTORY. 



prevented. The valve between the stout-chambered left ventricle and auricle does not present this 

 structure, but is divided into two or three lobes attached to tendinous processes. At the origin of the 

 great vessels the pulmonary artery and the aorta there are three valves, semi-lunar in shape and 

 by name. And this last vessel, often having given off the coronary artery to the heart itself, is 

 curved to the right, and then passes, backwards to go down the body. The blood from the body is 

 collected into three large veins two anterior venae cavce and one posterior. 



The lymphatic system is well developed, and of the so-called "lymphatic hearts," which are well 

 known in the Frog, the posterior ones have been observed in some, and especially in the Ratite birds. 



The lungs, or organs in which the blood effects an exchange of its gases with the outer air, 

 are paired, and set on either side of the heart. As is elsewhere mentioned, the nostrils are not 

 provided with muscles, and there is no epiglottis sufficiently well developed to cover the entrance 

 into the long tube, or tracliea, which runs down the neck. This tube, which does not always 

 take a straight course, is essentially made up of a number of rings of cartilage, which are for the 

 greater part perfect, and not, as in man, imperfect rings. The bronchi which are given off from this 

 tube, to the right and left, have their rings imperfect, and they do not show that two-forked mode of 

 division which is so characteristic of mammals. The lungs are of a rosy colour, and of a compara- 

 tively small volume ; they are marked externally by depressions corresponding to the characters of the 

 vertebrae and ribs, to which latter they are firmly attached, and they are not divided into lobes ; in their 

 texture they are spongy ; the air-tubes are given off from them at right angles to the main air-passage ; 

 these run nearly parallel to one another, and contain in their walls the true tissue of the respiratory 

 organ. The air-tubes are also connected with the air-cells, which are arranged in so remarkable 

 a manner as to deserve a full account. 



They are found in all birds with the exception of the Apteryx, according to Professor 

 Owen. Our knowledge of . their existence is primarily due to the illustrious William Harvey, 

 while it is to the distinguished anatomist, John Hunter, that we owe our knowledge of 

 the very curious fact that these air-passages and sacs communicate also with the cavities of 

 some of the bones of the skeleton. Though these sacs are not by any means highly vascular, 

 or supplied with vessels to the same rich extent as are the lungs, they are nevertheless of 

 enormous importance to the bird ; thus, they diminish the specific gravity of the animal. For 

 example, taking a bird which weighs 1,600 grammes, and has a volume of 1,230 cubic centimetres 

 or a specific gravity of 1'30 (j~j) it has been calculated (Bert) that 200 cubic centimetres of air can 

 be introduced ; now these centimetres would weigh '22 of a gramme, so that the specific gravity of the 

 animal would be reduced to 1 -05 (j^^?) or ( J -^p). Again, the air which is taken into the lungs is, in 

 high-flying birds, often of an extremely low temperature ; but this air is not only brought into contact with 

 that of the lungs, but also with that which has been warmed in the abdominal cavity. And again, the 

 air is often very dry as it is for the Ostrich on the desert plains of Africa but the air from the air- 

 sacs contains a large amount of moisture. Of the proper air-sacs there are nine ; of these, four the 

 two anterior and the two posterior thoracic lie in the thorax (breast) proper ; three the right and 

 left cervical, and the sac between the clavicles lie in front of the thorax ; while the last two are found 

 behind it and in the abdomen. From all of these, with the exception of those within the thorax, com- 

 munications are, or may be, given off to the bones of the vertebral column, to the humerus, to the 

 bones of the thigh, and to the sternum and the ribs ; but there is no communication between these 

 sacs and the air-spaces which are so constantly found in the bones of the skull, and which are in con- 

 nection with the air-cavities of the ear and of the nose. The inter-clavicular sac has been observed to 

 be covered with a thick layer of muscle in those birds, at any rate, which perform somersaults, and 

 it has been suggested that this layer of muscle is capable of driving the air in the sac backwards. It 

 is obvious that such an operation would send the centre of gravity of the animal nearer the head, and 

 would, so far, be of assistance in the execution of the curious movement alluded to. 



It has been suggested that the air-sacs are of assistance in increasing the resonance of the bird's 

 voice. Be this as it may, attention must now be turned to the organ of voice. This organ may 

 take one of three forms, or, if absence is to be counted, four. There is no organ of voice in. 

 the Ratitse, or in the American Vultures (Cathartid(v). It is, when present, remarkable for being 

 developed at the lower, and not at the upper, end of the trachea ; while the true vocal cords, which, 



