THE ANATOMY OF BIRDS. — PNEUMATOLOGY. 
199 
Both limbs of birds have ii prime peculiarity of their arteries as compared with mammals. 
In the fore limb, the blood supply being chiefly absorbed by the immense pectoral nmscles, 
vessels which in mammals are small axillary branches appear like the main continuation of the 
suhclavian 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- 
responds to the twig which in man accompanies the great sciatic nerve (comes nervi ischiatici) ; 
and the rare human anomaly of a posterioi' 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, forniiug various plexus, — nodes, glands," or ^'lymph-hearts" in their coarse. 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 tivo (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 effete, which is still fit for feeding the blood. The lacteals contain 
chyle, — the other kind of lymph, drained off 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 of 
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 amoeba-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 with 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 combustion 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 effete 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 the proper respiratory office. 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-ceUs; some large ones run through the lung, pierce its surface (as at u, u, fig. 101), and end 
