CHAPTER V. 

 COLORING MATTERS. 



SOME of the animal tissues and fluids are distinguished, in addition 

 to their other features, by characteristic colors, due to the presence 

 of certain coloring matters. In some instances, as in the red globules 

 of the blood, and the green leaves of plants, the coloring matters are 

 directly connected with active physiological functions. In others, as 

 in the choroid coat of the eye, they are essential to the physical phe- 

 nomena of the organs to which they belong. But notwithstanding the 

 evident importance of these substances, and the striking character of 

 their optical properties, they are in many respects more difficult of study 

 than the other ingredients of the body. This is partly due to the com- 

 paratively small quantity in which they occur, and to the readiness with 

 which they are decomposed or altered in the process of separation ; and 

 it is sometimes difficult to decide whether a variation of tint be due to 

 the different proportions of several coloring matters or to the varying 

 degrees of concentration of a single one. 



The coloring matters are all nitrogenous compounds, but differ in 

 essential particulars from the albumenoid substances. Those which 

 have been most fully examined are known to be crystallizable ; and 

 it is probable that all of them might be obtained in a crystalline form, 

 could they be completely separated without decomposition. The most 

 remarkable of all, and that which possesses the most important physi- 

 ological properties in the animal body, is the red coloring matter of the 

 blood. It appears to be analogous in many respects to the green matter 

 of leaves and leaflike organs in the vegetable world. Each of these two 

 coloring matters is the most abundant and widely diffused in its own 

 kingdom, and is distinguished by the identity of its characters in many 

 different species of animals and plants respectively. While the red 

 coloring matter of the blood, on the one hand, is the agent by which 

 oxygen is absorbed and distributed in the animal body ; on the other, it 

 is the green coloring matter of plants by which carbonic acid and water 

 are decomposed and oxygen set free in the act of vegetation. It is 

 believed by many that all the coloring matters of the body, in man and 

 the vertebrate animals, are derived by transformation from the coloring 

 matter of the blood ; and although we have no complete proof that this 

 is true in all cases, yet it is evident that these substances have a close 

 physiological relation with each other, perhaps as distinct and real as 

 that between the various members of the albuminous or saccharine 



groups. 



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