290 GENERAL ORNITHOLOGY part ii 



blood. The machinery of circulation is two sets of vessels — the 

 hcematic, or vascular system proper, consisting of the heart, arteries, 

 veins, and capillaries, for the blood-circulation ; and the lymphatic, 

 consisting of lymph-hearts and vessels, for the flow of lymph. The 

 lymphatics, converging from all parts of the body, and especially 

 from the intestines, end in vessels which pour the lymph into the 

 veins of the neck. The heart is the central organ of the blood- 

 circulation, by which that fluid is pumped into all parts of the body 

 through the arteries or efferent vessels ; straining through the net- 

 work of capillaries, it returns to the heart through the veins, or 

 afferent vessels. The set of efferent vessels is the arterial system ; 

 that of afferent vessels is the venous system. The blood in arteries 

 excepting the pulmonary is bright red ; that in veins excepting the 

 pulmonary is dark red. The change from bright to dark occurs in 

 the capillaries of the system at large ; the change from dark to 

 bright only in the capillaries of the lungs and air-sacs. The systemic 

 blood circulation is completely separated from the piuhiwnic in all 

 animals in which, as in birds, the right and left sides of the heart 

 are separated from each other ; such circulation is said to be double ; 

 that is, arterial and venous blood only mingles in the capillaries, 

 whether of the lungs or others, and therefore at the periphery of the 

 vascular system : the heart being the centre of that system. Blood, 

 in all or some of its constituents, permeates absolutely every tissue 

 of the body. Those tissues whose capillaries are large enough for 

 the passage of all the constituents of blood are said to be vascular ; 

 those which only feed by sucking up certain constituents of the 

 blood, and have no demonstrable capillaries, are called non-vascular. 

 But nutrient fluid penetrates the densest tissue, as the dentine of 

 teeth ; no permanent tissues are really non-vascular, or they would 

 soon die, as do feathers, which require to be renewed once a year 

 or oftener. 



Lymph and the lymphatics are noticed further on. Blood con- 

 sists of water in which several ingredients are dissolved, and cer- 

 tain solid bodies are suspended. Its water is salted, albuminated, 

 fibrinated, and corpusculated. The proportions, which vary in 

 different birds and at different times in the same bird, are in round 

 numbers: Avater 80, fibrine and corpuscles 15, albumen and salts 

 5 ==100 parts. Withdrawn from the body and allowed to settle, 

 blood separates into two parts, serum and coagulum. The serum is 

 the clear yellowish salty albuminous water ; the clot is the fibrine, 

 in the meshes of which are mired the corpuscles, reddening the 

 whole mass. The plasma, j^lasm or plastic material of the blood, is 

 its substance dissolved in Avater ; that is to say, minus the solid cor- 

 puscles. These latter interesting little bodies are a myriad of 

 minute animals, Avhich swim in the life-current, and are named 



