SEC. IV ANATOMY OF BIRDS 295 



ischiac artery, the femoral being small. This ischiac artery corre- 

 sponds to the twig which in man accompanies the great sciatic nerve 

 (comes nervi ischiatici) ; and the rare human anomaly of a looakr'wr 

 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 accompany- 

 ing the main blood vessels, forming various pleoMS, — nodes, "glands," 

 or " lymph hearts," in their course. Superficial lymphatics, so promi- 

 nent in mammals, are little developed, though lymphatic glands are 

 found in the armpit and groin of some birds. These are the 

 systemic lymphatics ; a special set, the lacteals, arise by numberless 

 twigs in the course of the small intestine, uniting and reuniting 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 duets ; 

 which terminal trunks of the whole lymphatic system empty into 

 tlie right and left jugular veins at the root of the neck. The contents 

 of the vessels diff"er correspondingly. Pure lymph is a pale, limpid, 

 albuminous fluid, containing when maturely elaborated a number of 

 irregular amoeboid bodies indistinguishable from the white formative 

 corpuscles of the blood (p. 291). It is strained out of the tissues at 

 large, being that material, not yet efi'ete, which is still fit for feed- 

 ing the blood. The lacteals contain chyle, — the other kind of lymph, 

 drained off" by the mucous meml^rane of the intestine from the pre- 

 pared 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. Pneibiatology : the Eespiratory Systeji 



The Organs of Respiration provide for the ventilation of the 

 body. Since the respiratory process is also calorific, they likewise 

 furnish a heating apparatus. They consist essentially of air-passages 

 and air-spaces connected with lung-tissue, being therefore ^yulmonary 

 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 ; purification of the blood and warming of the body being 

 two inseparable results obtained. Dark blood flowing to the lungs. 



