60 BIOLOGY. [Book i. 



yi&lding incessantly materials to tlie exterior world and to the 

 tissues, and incessantly taking them back, having a temperature 

 nearly equal, as long as that of the exterior medium does not 

 suffer very great oscillations. For the anatomical elements, the 

 plasmas are a veritable living atmosphere. 



4:.— Of the Blood. 



In the vertebrates the most important of the plasmas, the 

 blood, is a red sap unceasingly circulating with greater or lesser 

 activity. In the invertebrates the blood is animated by a much 

 slower movement of translation. It is contained in apparatus 

 not so well constructed, and is generally colourless and trans- 

 parent like lymph. Often even the blood and the lymph of the 

 invertebrates are confounded ; where there are distinct san- 

 guineous cavities and lymphatic cavities, as happens in some 

 annelate worms, the blood is tinted with a special colour — it is 

 sometimes red, sometimes yellow, green, violet, or bluish. The 

 sanguineous cells which it sometimes in that case conveys, are 

 almost always colourless. However, there are sanguineous, 

 coloured globules in the terebella and the cephalopods.^ 



The blood of the vertebrates, with which we have especially 

 to occupy ourselves, is, according to a felicitous comparison of 

 01. Bernard,^ an interior medium in which live the anatomical 

 elements as the fishes live in the water. These anatomical 

 elements, moreover, retain in the blood their physiological inde- 

 pendence, and though drawing their nutritive materials from the 

 sanguineous plasma, they do, not allow themselves to be imbibed 

 by it, which is an essential condition of endosmotic exchanges. 

 Oonsequently we see, for example in the blood of the vertebrates, 

 potash dominating in the globules, and soda in the plasma. 



The physical qualities of the blood of the superior vertebrates 



1 R. Wagner, quoted by Fr. Leydig {Traiti d'Histologie de fSomme et <ks 

 Animaux, p. 509, Paris, 1866.) 



" CI. Bernard, Lepms sur les Propriillis des Tissus vvoants, pp. 5.5-53. 

 Paris, 18C6, Svo. 



