196 GENEBAL OBNITHOLOGY. 



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 mingle 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 consists of water in which 

 several iugi'edients are dissolved, and certain solid bodies are suspended. Its water is salted, 

 albnminated, fibrinated, and corpusculated. The proportions, which vary in different birds and 

 at different times in the same bird, are in round numbers : water 80, fibrine and corpuscles 15, 

 albumen and salts 5 = 100 parts. Withdrawn from the body and allowed to settle, blood sepa- 

 rates 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, plasm or plastic material of the blood, is its substance dissolved 

 in water : that is to say, minus the solid corpuscles. These latter interesting little bodies are a 

 myriad of minute animals, which swim in the life-curreut, and are named Hcematamosba 

 cruentata. They have been supposed to be of two species ; but the so-called white blood 

 corpuscles, or leucocytes, indistinguishable from lymph corpuscles, are simply the forma- 

 tive stages of the red blood-discs. In its early colorless stage, the Hcematamoeba is a 

 nucleated mass of protoplasm (protoplasm is the indifferent substance out of which all animal 

 tissue is derived), of no determinate size or shape, exhibiting active amoeboid movements. 

 Later in the life of the minute creature, it passes into a sort of encysted state, in which it red- 

 dens and acquires definite dimensions and configuration. In birds, these " blood -discs " are 

 flat, elliptical, and nucleated, that is, containing a kernel ; they average in the long diameter 

 ^OQ, in the short ^o' *^f '^^ ^^^^- Thus they differ decidedly from the flat, circular, non- 

 nucleated, red blood-discs oi Mammalia, which latter are supposed to be rather //'ee nuclei than 

 perfected Hcematamoebfe. The red color of blood is entirely due to the presence of these 

 uuicellular animals. The energy of respiration, and corresponding activity of circulation in 

 birds, make them hcematothermal, or hot-blooded; the pulse is quickest, the blood hottest, 

 and richest in organic matter, in these of all animals. 



The Heart is a hollow muscular organ, at the physiological centre of the haematic vas- 

 cular system. Its muscle presents the principal exception to the rule, that the contractility of 

 Myamceba striata (see p. 192) is subject to voluntary control. It is the most industrious organ 

 of the body, never ceasing its rhythmic systole and diastole, or contraction and dilatation, from 

 the moment of the first pulsation in the coutractile vesicle which begins it, to that when the 

 "muffled drum" gives the last beat of the "funeral march to the grave." The arteries are 

 the elastic thick-walled branching tubes which leave the heart on their way to the body at 

 large ; their pulsations, over which the vaso-motor nervous system presides, are isochronous 

 with the heart-beats, and arterial blood thus flows in jets. The veins are the vessels converg- 

 ing from all parts ; tliin-walled, less elastic, with more equable current. The capillaries are 

 the communicating vessels, of such size as just to permit the Haematamoebas to pass through ; 

 their network represents the terminations of arteries and the commencements of veins. The 

 heart in adult birds is completely double ; i. e., the right and left sides are perfectly separated. 

 It is also completely four-chambered ; i. e., there is an auricle and a ventricle on each side, 

 which communicate ; in embrvonic life the two auricles communicate by the foramen ovale, 



