196 
GENERAL 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 ingredients are dissolved, and certain 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 : water 80, fibrine and corpuscles 15, 
albumen acid 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 Withdrawn from the body and allowed to settle, blood sepa- 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 smm in the life-current, and are named Hcematamoeha 
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 aU 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 loug diameter 
■2tVo' short ywq'qi inch. Thus they differ decidedly from the flat, circular, non- 
nucleated, red blood-discs oi Mammalia, which latter are supposed to be rather /ree nuclei than 
perfected Hcematamoebce. The red color of blood is entirely due to the presence of these 
unicellular animals. The energy of respiration, and corresponding activity of circulation in 
birds, make them hcematothermdl, 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 
Myamoeha 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 contractile 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 -waUed 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 ; thin-walled, less elastic, with more equable current. The capillaries are 
the communicating vessels, of such size as just to permit the Hsematamoebas 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 embryonic life the two auricles communicate by the foramen ovale, 
