6 THE BLOOD. 



changing from black to red. In its passage through the capillaries of the system, in the 

 ordinary processes of nutrition, it loses oxygen and gains carbonic acid, changing from 

 red to black. During the intervals of secretion, the glands receive just enough blood 

 for their nutrition, and the ordinary interchange of gases takes place, with the con- 

 sequent change of color ; but, during their functional activity, the blood is supplied 

 in greatly-increased quantity, in order to furnish the watery elements of the secretions. 

 Under these circumstances, it does not lose oxygen and gain carbonic acid in any great 

 quantity, as has been demonstrated by actual analysis, and consequently there is no 

 marked change in color. When filaments of the sympathetic are divided, the vessels 

 going to the part are dilated, and the supply of blood is increased to such an extent, 

 that a certain proportion passes through without parting with its oxygen (a fact which 

 has also been demonstrated by analysis), and consequently it retains its red color. The 

 explanation in cases of syncope is probably the same, although this is merely a suppo- 

 sition. Even during secretion, a certain quantity of carbonic acid is formed in the 

 gland, which, according to Bernard, is carried off in solution in the secreted fluid. 



It may be stated, then, in general terms, that the color of the blood in the arteries is 

 bright red ; and, in the ordinary veins, like the cutaneous or muscular, it is dark blue, 

 almost black. It is red in the veins coming from glands during secretion, and dark during 

 the intervals of secretion. 



Anatomical Elements of the Blood. 



In 1661, the celebrated anatomist, Malpighi, in examining the blood of the hedgehog, 

 with the imperfect lenses at his command, discovered little floating particles which he 

 mistook for granules of fat, but which were the blood-corpuscles. He did not extend his 

 observations in this direction; but, a few years later (1673), Leeuwenhoek, by the aid 

 of simple lenses of his own construction, ranging in magnifying power from forty to one 

 hundred and sixty diameters, first saw the corpuscles of human blood, which he minutely 

 described in a paper published in the Philosophical Transactions, in 1674. To Leeuwen- 

 hoek is generally ascribed the honor of the discovery of the blood-corpuscles. 1 About a 

 century later, William Hewson described another kind of corpuscles in the blood, which 

 are much less abundant than the red, and which are now known under the name of white 

 globules, or, as they have been called by Eobin, leucocytes. 



Without following the progress of microscopical investigations into the constitution 

 of the blood, it may be stated that it is now known to be composed of a clear fluid, the 

 plasma, or liquor sanguinis, holding certain corpuscles in suspension. These corpusclef 

 are as follows: 



1. Eed corpuscles; by far the most abundant, constituting a little less than one-half 

 of the mass of blood. 



2. Leucocytes, or white corpuscles ; much less abundant, existing only in the pro- 

 portion of one to several hundred red corpuscles. 



3. Granules ; exceedingly minute, called, by Milne-Edwards, globulins, and, by Kolli- 

 ker, elementary granules. These are few in number, and are probably fatty particles 

 from the chyle. They are to be regarded as accidental constituents of the blood. 



Eed Corpuscles. -These little bodies give to the blood its red color and its opacity. 

 They are true, organized structures, containing organic nitrogenized and inorganic ele- 

 ments molecularly united, and, as an exception to the general rule, a little fatty matter 

 in union with the organic principles. They constitute a little less than one-half the mass 



1 Some writers give the credit of the discovery of the blood-corpuscles to Swammerdam. In 1658, Swammerdam 

 studied the blood-corpuscles of the frog and described them very accurately; but his researches were not published 

 until 1738, a number of years after his death. In questions of priority, it is usual to date discoveries from the time 

 of their first publication. 



