CLASSIFICATION OF THE TISSUES. 



25 



cells originate ? First, 

 is, from the lymphatic 



Fig. 32. — The blood current in tSe web of 

 the frog ; a, the vessel ; b, the epithelial cells 

 of the lissue. 



But whence do our lymphoid 

 from the lymph and chyle, that 

 glands, then from the spleen 

 and bone marrow. They are 

 carried away from both the 

 latter parts by the blood cur- 

 rent. 



What becomes of our cells 

 in the veins ? 



They become, in part, grad- 

 ually transformed into red 

 blood corpuscles, and cover 

 the loss of the latter. Whether, 

 however, a greater or only a 

 lesser portion undergoes this 

 metamorphosis, we are not 

 yet able to say ; for this, we 



must first learn more accurately the duration of the life of the 

 red blood corpuscles. 



The manner of this metamorphosis we can state in some 

 degree. The globular form changes to the specific one of the 

 red blood cell, and the protoplasma is replaced by a homo- 

 geneous colored substance. In mammalia and man, finally, 

 there is also a loss of the nucleus. 



Isolated examples of such intermediate forms have been 

 recognized in the blood for years, especially in that of the 

 spleen, the mammary ducts and the bone medulla. 



The bright red color of the arterial, and the dark of the 

 venous blood is caused by a combination of oxygen with the 

 haemoglobin, or a reduction of the latter. Prolonged changes 

 in the form of the blood corpuscles likewise exert a modifying 

 effect on the color. Distended, they lend a darker color to 

 our fluid ; shriveled, a brighter one. 



When a drop of blood is left to itself, it coagulates. The 

 filiform separation of the fibrine is shown in our Fig. 28, d. 



When blood is beaten, that is, the fibrine caused to coagu- 

 late, the cells sink, the red ones more rapidly, the colorless, 



