OF THE BLOOD. 43 



dog, rabbit, and some others, their diameter is from the 

 three-thousandth to the four-thousandth part of an inch. 



In birds, the globules are larger than 



in mammals. In the reptiles and ba- & 



trachia they are still larger. In the 

 proteus they attain their maximum. 



Finally, in fishes, the blood globules 

 are intermediate between those of birds 

 and the batrachia. 



Moreover, the blood globules are al- 

 ways flattened, and present a central 

 spot or nucleus, surrounded with a rim 

 or border. Their structure is extremely 

 difficult to be clearly made out; but 

 when seen to most advantage, they seem 

 to be composed of a central nucleus and 

 an envelope, resembling a bladder. This j^g. 28* 



envelope being depressed, gives to the 

 globule the appearance of a disc, swollen in the middle. It 

 is of a reddish colour, and seems formed of a substance re- 

 sembling jell} r , but very elastic. The central nucleus is of a 

 spheroidal form, and is not coloured. In mammals, the 

 nucleus is not distinct, and the central portion is depressed ; 

 but analogy induces us to suppose that, as in other animals, it 

 is also present in man. 



Other globules, spherical and colourless, exist in the blood, 

 resembling greatly those observed in the chyle ; from being 

 mingled with the red globules they are not readily observed. 



82. In the white blood of the invertebrate kingdom, 

 globules are also found, but different from those described; 

 the size varies much in the same individual, and their surface 

 has a raspberry appearance ; their form is generally spherical, 

 but neither a central nucleus nor external envelope is to 

 be seen. 



83. Composition of the Blood. The composition of the 

 blood is very complex In the higher animals we find water, 

 albumen, fibrin, a colouring matter containing iron, a yellow 

 colouring matter, several fatty substances, as cholesterine, cere- 

 brine (a substance containing phosphorus) ; many salts, 

 as chloride of sodium or sea salt, sulphate of potass, 



* Fig. 28. Elliptic globules of the blood in birds, batrachia, and fishes ; 

 a, globules of the blood in the domestic fowl, seen in profile ; b, globules 

 of the blood in the frog ; c, globules of the blood in a fish of the shark kind 

 (equally magnified) . 



