The temperature of the blood of the bird is, in requirement with the conditions of its existence, hot—that is to say, it is ordinarily hotter than the temperature of the surrounding air, and is found to register between 100° (Gull) and 112° (Swallow) on Fahrenheit’s scale, or from two to fourteen degrees more than does that of man. Birds and mammals, are, speaking broadly, the only hot-blooded animals now existing, and it has consequently been suggested that they should be grouped together as such, in opposition to the rest of the Vertebrata. But it is obvious that this character of the temperature is merely dependent on physiological conditions; and were this a treatise on the anatomy of birds rather than one on their natural history, the statement of this fact would not receive the prominence here given to it. The high temperature of any body may be preserved from cooling influences by two methods: thus, tea in a well-polished silver teapot keeps hot because the rays of heat are but slightly radiated from its surface; or a less costly teapot may be kept hot by covering it with a loosely-fitting “cosy,” which, being made of badly-conducting materials, “keeps the heat in.” It is, then, clear that the heat of a body is best preserved when it is covered by a bad radiator and a bad conductor of heat; and this is just the case with birds: the polished feathers are bad radiators, and the air entangled among them forms a bad conductor.
The blood corpuscles are, broadly speaking, about twice as large as in man; those which are coloured red are oval in shape, as they are in nearly all of the lower Vertebrates and in the Camels among mammals. Like the white ones, they are “nucleated.” The heart is, as in mammals, divided into four chambers. It is a condition of the circulation in hot-blooded and rapidly-breathing animals that the current of arterial blood from the heart, and the current of venous blood to it, should be kept as much as possible separate; no reflection is needed to show that the blood freshly purified by contact with the air in the lungs must be kept as distinct as can be from the blood which has lost its purity in passing through the body; in other words, it is required that there should be a similar result in birds and in mammals.
Birds, like all warm-blooded creatures, have the heart divided into four cavities—two ventricles and two auricles—those of the right side being completely separated from those of the left. The whole is enclosed in a pericardium, a thin, but strong, membrane. The right ventricle has thin muscular walls, and almost completely envelopes the left. The right auricle has a remarkable valve in the shape of a fleshy leaflet, which appears almost to be a portion of the inside of the ventricle that has become detached from the partition between the two ventricles. The blood, under certain circumstances, passes between this septum, or partition, and the leaflet, into the auricle; but when the beat of the heart takes place (the systole), the septum, being convex, is forced against the leaflet on the other side of the auricolo-ventricular opening, and the passage of the blood, through this, is 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 venæ cavæ 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 trachea, 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 comparatively small volume; they are marked externally by depressions corresponding to the characters of the vertebræ 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 (16001230) 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 (1600+0·221230+200) or (1600·221520). 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, communications 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 connection 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 Ratitæ, or in the American Vultures (Cathartidæ). 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, by their vibration produce the notes of the human voice, are altogether and always absent from the larynx; in other words, the vocal organ is not the larynx, but an organ seated at a lower level, and known as the syrinx. This instrument may, further, be formed in the trachea alone (as in some American Passerines), or in the bronchi alone (as in Steatornis), or at the point at which the tracheal and bronchial tubes pass into one another (as in the majority of singing birds).
The last-mentioned, or bronchio-tracheal syrinx, consists of the following parts; (i.) a tympanic chamber formed by the union of some of the lower rings of the trachea; (ii.) a membranous septum separating from one another the tracheal orifices of the two bronchi; (iii.) on either side a tympaniform membrane, formed on the inner side of the uppermost bronchial rings; in consequence of this these bronchial rings are not complete circles; their mucous membrane is developed into a fold which bounds one side of a cleft which is formed by the presence on the other side of the above-mentioned tympaniform membrane. The air which passes through these bronchial clefts sets in vibration the membranes which bound them, while the character of the note is affected by the position of the bronchial half-rings, and the length of the column of air in the trachea. These rings have their positions changed by five lateral muscles, which act on their ends, and so rotate them. The principle variations in the characters of the muscular supply of the organ of the voice were long ago worked out by Johannes Müller, the famous German anatomist and physiologist.
It is also to this observer that we owe our first information with regard to the bronchial syrinx of Steatornis; the anatomy of this animal was also investigated by the late Prof. Garrod, who gave the following account of its vocal apparatus:—“Each semi-syrinx, as it may be termed, is formed on the same principle as that of the combined organ in most of the non-singing birds. Taking for description that of the left side, it is found that the thirteenth bronchial ring is complete, though considerably flattened from side to side; the fourteenth is not complete in the middle of its upper surface; it is a little longer from before backwards than the one above, and not so long as the one following it. The fifteenth is only a half ring, its inner portion being deficient; it is slightly convex upwards, and articulates, both at its anterior and posterior ends, with the fourteenth incomplete ring and the sixteenth half-ring. The sixteenth half-ring is concave upwards, and so forms an oval figure in combination with the one above, which is filled with a thin membrane to form part of the outer wall of the bronchus. There is a membrane also between the ends of these and the succeeding half-rings, which completes the tube of the bronchus internally.”
