CHAP. II. J THE BLOOD. 71 



partly in a state of solution and partly in the form of feeble chemical 

 compounds. 



Whilst the oxygen which is contained in the mixed gases is derived 

 wholly from the decomposition of the oxy-haemoglobin of the blood 

 corpuscles, the greater part of the carbonic acid and the whole of the 

 nitrogen are derived from the plasma, in which they exist mainly in a 

 state of simple solution, though the carbonic acid is in part in a state 

 of feeble combination, probably in the form of sodium hydric carbo- 

 nate, NaHCO,. 



SECT. 4. THE COLOURED CORPUSCLES OF THE BLOOD. 



Shape, size, The red colour of the blood of vertebrates is due to 



&c. of the suspension in a colourless or slightly coloured liquid 



coloured O f large numbers of solid bodies, of which the principal 



sorpuscies. solid ingredient is a red colouring matter, haemoglobin. 



In the blood of man and the mammalia generally 1 , the coloured 

 blood corpuscles are non-nucleated biconcave disks, whilst in the blood 

 of birds, reptiles, and most fishes they are nucleated, elliptical, bicon- 

 vex bodies. 



The size of the coloured blood corpuscles of the various orders of 

 mammals varies somewhat, though with some exceptions not within 

 very wide limits. The red blood corpuscles of man are amongst the 

 largest, being for instance larger than those of any domestic animal 

 inhabiting Europe. 



The diameter of the average red blood corpuscles of human blood 

 is about the y^th of a millimetre, or about 7'9/ 2 , and the thickness 

 about 1'8/Aj expressed in English measurements the average coloured 

 blood corpuscle measures about s^^th f an inch in diameter and 

 about y^oijth f an mcn in thickness (Gulliver) 3 . 



By means of a method which could only yield rough ap- 

 proximations to the truth, Welcker 4 determined the approximate 



MM. Cube 



volume of a human coloured blood corpuscle to be 0-000,000,072 or 

 seven ten-millionths of a cubic millimetre, and the approximate 

 superficial area to be about 0'000128 or rather more than one ten- 

 thousandth of a square millimetre. 



1 In the blood of the Camelidae the red corpuscles are oval. 



2 The Greek letter /* is now employed to represent the micro-millimetre, or 1000th 

 part of a millimetre, which is taken as a convenient unit for microscopic measurement. 

 The micro-millimetre corresponds to about T^mnnr^ 8 of an English inch or more 

 accurately to 0-00003937 of an inch. 



3 By far the most complete set of measurements of the corpuscles of the verte- 

 brata was made by Mr Gulliver, and the results of his researches were collected 

 together and published as a note (cxvin) to his edition of Hewson's works. They are 

 transcribed, with some additions, by Milne-Edwards, Lemons sur la Physiologie, Vol. i. 

 p. 84 et seq. 



4 Welcker : for an account of his method followed see Strieker's Human and Com- 

 parative Histology, Vol. i., Article "Blood," by Rollett, p. 383. 



