PHYSICAL CHAEACTEES. 107 



continues to flow, the blood in these vessels will be bright 

 red ; but when secretion becomes arrested, as it soon does 

 after exposure of the organs, it presents no difference from 

 the blood in the vena cava. In the sub-maxillary gland, by 

 the galvanization of a certain nerve, which he calls the motor- 

 nerve of the gland, Bernard has been able to produce secre- 

 tion, and by the galvanization of another nerve, to arrest it ; 

 in this way changing at will the color of the blood in the 

 vein. It has been found by the same observer that division 

 of the sympathetic in the neck, which dilates the vessels and 

 increases the supply of blood to one side of the head, produces 

 a red color of the blood in the jugular. He has also found 

 that paralysis of a member by division of the nerve has the 

 same effect on the blood returning by the veins. 1 



The explanation of these facts is evident when we reflect 

 upon the reasons why the blood is red in the arteries and 

 dark in the veins. Its color depends upon the corpuscles ; 

 and as the blood passes through the lungs it loses carbonic 

 acid and gains oxygen, 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 secre- 

 tion, the glands have just enough blood sent to them for their 

 nutrition, and the ordinary interchange of gases takes place, 

 with the consequent change of color ; but during their func- 

 tional 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, 2 and consequently 

 experiences no change in color. When filaments of the sym- 

 pathetic are divided, the vessels going to the part are dilated, 

 and the supply of blood is increased to such an extent, that a 



1 BERNARD, op. cit. 



2 Unpublished lectures delivered by Bernard in the College of France during 

 the summer of 1861. 



