THE ANATOMY OF BIRDS. — PNEUMATOLOGY. 199 



Both liinbs of birds have a prime peculiarity of their arteries as compared with mammals. 

 In the fore limb, the blood supply being chietiy absorbed by the immense pectoral muscles, 

 vessels which in mammals are small axillary branches appear like the main continuation of the 

 subclavian trunk, and the brachial arteries are correspondingly reduced. In the leg, the main 

 source of supply is the great ischiac artery, the femoral being small. This ischiac artery cor- 

 fesponds to the twig wliich in man accompanies the great sciatic nerve (comes nervi ischiatici) ; 

 and the rare human anomaly of a posterior main vessel of the thigh is therefore a reversion 

 (atavism) to the avian rule. There is no single proper renal artery to the kidney. 



The Lymphatics of birds consist chiefly of a deep set accompanying the main blood- 

 vessels, forming various pZeJSWs, — nodes, "glands," or "lymph-hearts" in their course. Su- 

 perficial lymphatics, so prominent in mammals, are little developed, though lymphatic glands 

 are found in the arm-pit and groin of some birds. These are the systemic vessels ; a special 

 set, the lacteals, arise by numberless twigs in the course of the small intestine, uniting and re- 

 uniting to form at length two (not one as in mammals) main tubes, which lie along either side 

 of the spinal column. These are the thoracic ducts; which terminal trunks of the whole lym- 

 phatic system empty into the right and left jugular veins at the root of the neck. The contents 

 of the vessels differ correspondingly. Pure lymph is a pale, limpid, albuminous fluid, contain- 

 ing when maturely elaborated a number of irregular amoeboid bodies, indistinguishable from 

 the white formative corpuscles of the blood (p. 196). It is strained out of the tissues at large, 

 being that material, not yet eflfete, which is stiU fit for feeding the blood. The lacteals contain 

 chyle, — the other kind of lymph, drained ofi" by the mucous membrane of the intestine from the 

 prepared food in that tube ; an albuminous fluid, milky or cloudy from the abundance of oil- 

 globules, which, after mingling with the systemic lymph, is poured directly into the current t)f 

 the blood, in the manner above said. Since the lacteals do not appear to begin with open 

 mouths, the chyle must soak into them through the lining membrane of the intestines ; and 

 as this consists of a layer of amceba-like animals, through whose bodies the chyle passes, it is 

 quite true to say that the whole organism is nourished upon the excrement of amoebas. 



e. Pneumatology : the Respiratory System. 



The Organs of Respiration provide for the ventilation of the body. Since the respira- 

 tory process is also calorific, they likewise furnish a heating apparatus. They consist essen- 

 tially of air-passages and air-spaces connected with lung-tissue, being therefore pulmonary 

 organs. No other animals are so thoroughly permeated as birds \vith the atmospheric medium 

 in which they live ; in no others are the respiratory functions so energetic and effectual. The 

 lung may be likened to a blast-furnace for the cijmbustion of decayed animal matter ; purifica- 

 tion of the blood and warming of the body being two inseparable results obtained. Dark 

 blood flowing to the lungs, heavy with efiete carbonaceous matters, is there relieved of its bur- 

 den and aerated by the action of oxygen ; the products of combustion being exhaled in the 

 form of carbonic dioxide and water. Aside from the proper lung-tissue, the capillary substance 

 of the immense air-sacs tends to the same result. There is likewise, in birds, a lesser system 

 of ventilation, by which air is admitted to cranial bones through the eustachian tubes ; but 

 this is unconnected with tlie proper respiratory oflSce. Pulmonary tissue consists chiefly of a 

 wonderful net (a rete mirabile) of capillaries, interlacing in every direction, bound together and 

 supported by fine connective tissue, and invested with membrane so delicate that their walls 

 seem naked, their exposure to the air being thus very thorough. Air gains such intimacy 

 with the capillaries through the larynx, trachea (fig. 101, o), and bronchial tubes (r, r), these 

 being the primary air-passages. But all the bronchial tubes do not subdivide into the ultimate 

 air-cells; some large ones run through the lung, pierce its surface (as at u, u, fig. 101), and end 



