This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Fig. 359. Digestive apparatus of a Fowl.
(2059). The spleen (fig. 359,f) is of very small size in all birds; it is situated near the anterior extremity of the pancreas, and is loosely connected to the side of the proventriculus (b.) The distribution of its vessels and its general structure are the same as in Mammalia.
(2060). The lymphatic system is well developed; and the course of the lymphatic vessels has been investigated with great care by various anatomists. The vessels themselves are thin, and have but few valves; they principally accompany the larger blood-vessels from all parts of the body to the aorta, around which they form a plexus, and ultimately join to give rise to two principal trunks or thoracic ducts; these terminate severally in the right and left jugular veins, and into these vessels the greater proportion of the lymph and chyle absorbed is of course poured, to be mixed with the circulating blood.
(2061). Before describing the circulatory apparatus of Birds, it will be advisable in the next place to consider the nature and disposition of their organs of respiration, which, from what has been already stated Respiratory System. concerning the heat and purity of the blood in these creatures, prepared to find presenting the highest possible condition of develop ment. Birds, in fact, breathe not only with their lungs, but the vital element penetrates every part of the interior of their bodies, bathing the surfaces of their viscera and entering the very cavities of their bones; so that the blood is most extensively subjected to its influence. The lungs, in fact, are no longer closed bags as those of Reptiles are, but rather resemble spongy masses, of extreme vascularity, firmly bound down in contact with the dorsal aspect of the thorax, - their posterior surface being fixed to the ribs on each side of the vertebral column, and entering deeply into the intercostal spaces. Such lungs are obviously incapable of alternate dilatation and contraction; so that inspiration and expiration must be provided for by a mechanism specially adapted to the emergency.
From an examination of fig. 360, the arrangement adopted will easily be understood. The bronchi derived from the bifurcated inferior extremity of the trachea plunge into the anterior face of the lungs (c c), and by innumerable canals distribute air throughout their spongoid substance; but the main trunks of the bronchial tubes, passing right through the pulmonary organs, open by wide mouths, represented in the figure, into the cavity of the thorax, into which the air likewise freely penetrates. The whole thoracico-abdominal cavity is moreover divided by septa of serous membrane into numerous intercommunicating cells, all of which are freely permeated by the atmospheric fluid, which in most instances is admitted into the very bones themselves, and even penetrates to the interspaces between the muscles of the neck and limbs, thus, in some birds of powerful flight, gaining free access to almost every part of the system.
Fig. 360. Inferior larynx and lungs of a bird.
(2062). The mechanism by which the air is drawn into, and then expelled from, this extended series of respiratory cells is sufficiently simple, the whole being accomplished by the movements of the expanded sternum, assisted slightly by the abdominal muscles. The descent of the sternum from the vertebral column necessarily enlarges the capacity of the chest, and, acting like a great bellows, sucks in air through the trachea, which not only fills all the spongy substance of the lungs, but penetrates to all parts whereunto air is admitted; while the ascent of the sternum, and consequent contraction of the thoracico-abdominal space, alternately effects its expulsion.
(2063). The results obtained by this unusual arrangement are of great importance in the economy of the feathered races. In the first place, the perfect oxygenization of the blood is abundantly secured. Secondly, from the high temperature of the blood, the air drawn in becomes greatly rarefied, and thus materially diminishes the specific gravity of the bird. Thirdly, from the inflation of the whole body, the muscles, more especially those of flight, act with better leverage and firmer purchase; so that their efforts are materially favoured. And, lastly, it is owing to the capacity of the air-cells that the Singing Birds are enabled to prolong their notes to that extent which renders them pre-eminent among the vocalists of creation.
(2064). In connexion, therefore, with the respiratory system of the feathered races, it will be advisable, in the next place, to consider the construction of the air-passages whereby the atmospheric fluid passes into and out of the body, and more especially of the organs of voice connected with them.
(2065). The trachea is of very great proportionate length in correspondence with the elongated neck - commencing at the root of the tongue, and extending into the thoracic cavity, where it divides into two bronchial tubes, one appropriated to each lung (fig. 360, 11.) The trachea of Birds is composed of cartilaginous rings, which are very generally ossified, - each ring, with the exception of two or three immediately beneath the upper larynx, forming a complete circle (fig. 361, a) surrounding the tracheal tube: these rings are enclosed between the soft membranes of the trachea, and thus keep the air-passages constantly-permeable to the atmosphere.
(2066). In many birds, especially among the web-footed tribes, the trachea suddenly dilates into wide chambers or cavities of different forms and dimensions - a circumstance the object of which has not as yet been satisfactorily explained; and, what is still more inexplicable, in some genera, and those too with the longest necks, as, for example, the Wild Swan and many of the Wading birds, the lower part of the trachea is lengthened out and variously contorted before it terminates in the chest. This long trachea is provided with muscles whereby the rings may be approximated; and thus the length of the tube is considerably modified: these muscles (fig.360, a,b,h) arise from the sternum, and sometimes also from the furcula, and are continued along the sides of the windpipe throughout its whole length.