196 GENERAL ORNITHOLOGY. 



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 periplieni 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 enoush 

 for the passage of all the constituents of blood are said to be vnscidar: those which only feed by 

 sucking up certain constituents of the blood, and have no demonstrable capillaries, are called 

 non-vascular. But nutrieut tiuid 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 iugredients are dissidved. and certain solid bodies are suspended. Its water is salted, 

 albuminated, fibriuated, and coifiusculated. The proportions, which vary in diti'erent birds and 

 at difl'erent times in the same bird, are in round numbers : water 80, fibrine and corpuscles 1.5, 

 albumen aud salts .5^100 parts. Withdrawn fmui the body and allowed to settle, blood sepa- 

 rates into two parts, serum and coaijuhiiii. The serum is the clear yellowish salty albuminous 

 water ; the clot is the fibrine. in the meshes of Avhich are mired the corpuscles, reddening the 

 Avhole mass. The plasma, plasm or plastic material of the blood, is its substance dissolved 

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

 myriad of minute animals, which swim iu the life-current, and are named Ha:)iiatamwba 

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

 corpuscles, or leucocijtes, indistinguishable from lymph coqiuscles, are simply the forma- 

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

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

 tissue is derived), of no deterndnate 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 rod- 

 dens and acquires definite dimensions and configuratiim. In birds, these " blood-dises " are 

 flat, elliptical, and nucleated, that is, containing a kernil : they average iu the long diameter 

 ■jj^riy, in the short ^-jViT' "^ '^" i'^<^'''- Thus they difler ilecidedly fnun the flat, circular, non- 

 nucleated, red blood-discs i>{ Slinnmalia, which latter are suiiposeil to be vather free nuclei than 

 perfected Hccmatamo'bic. The red cidor of blood is entire ly (Uie to the presence of these 

 unicellular animals. The energy of respiration, and corresiionding activity of circulatiou in 

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

 and richest iu organic nuitter, iu tliese of all aniuurls. 



TUe Heart is a hollow muscular organ, at the physiidogical centre of the htematic vas- 

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

 Mijamaha striata (see p. 19:2) is subject to voluntary control. It is the most industrious organ 

 of the body, never ceasing its rhjihrnie sijstole aud diastole, or contraction and dilataticni, from 

 the moment of the first pulsation iu the contractile vesicle which begins it, to that when the 

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

 the elastic thick-walled branching tubes which leave the he:n-t on their way to the body at 

 large ; thilr pulsations, over which the vaso-motor nervous system |n'esides, are isochronous 

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

 ing from all parts ; thin-walled, less elastic, with more equable cun'eut. The capillaries are 

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

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

 heart in adult hir<ls is completely double ; t. c, 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, 



