SEC. IV AArJTOJ/V OF BIRDS 291 



Hcematamceba cruentata. They have been supposed to be of two 

 species ; but the so-called white blood corpuscles, or leucocytes, in- 

 distinguishable from lymph corpuscles, are simply the formative 

 stages of the red blood-discs. In its early colourless stage, the 

 Hcematamceba is a nucleated mass of protoplasm {])rotoplasm 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 Avhich it reddens and acquires definite dimen- 

 sions and configuration. In birds these " blood-discs " are flat, 

 elliptical, and nucleated, that is, containing a kernel ; they average 

 in the long diameter -^-^jj, in the short, Tj-g-Vrr? of an inch. Thus 

 they differ decidedly from the flat, circular, non - nucleated, red 

 blood-discs of Mammalia, which latter are supposed to be rather free 

 nuclei than perfected Hcematamcehce. The red colour of blood is en- 

 tirely due to the presence of these unicellular animals. The energy 

 of respiration, and corresponding activity of circulation in birds, 

 make them Juemaiothermal, 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 vascular system. Its muscle presents the 

 principal exception to the rule, that the contractility of Myamcela 

 striata (see p. 285) is suljject 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-walled, branching 

 tubes which leave the heart on their way to the body at large ; 

 their pulsations, over which the vasomotor nervous system presides, 

 are isochronous with the heart-beats, and arterial blood thus flows 

 in jets. The veins are the vessels converging 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 Haema- 

 tamcebas to pass through ; their network represents the termina- 

 tions 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-chambei"ed ; 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, 

 which afterward closes. Arteries proceed from the strong muscular 

 ventricles ; veins are received by the weaker auricles. The course 

 of the blood is : From the body, excepting the lungs, it comes dark 

 and heavy with products of decomposition, through the caval veins 



