86 NORMAL HISTOLOGY. 



infrequently in normal blood very often under 

 pathological conditions red blood-cells are seen 

 which are much smaller than the above-described 

 forms, and are often spheroidal in shape. 



The red blood-cells owe their color, as well as 

 their capacity for performing certain important 

 physiological functions, to the presence in them of 

 a crystallizable substance called hcemoglobin. The 

 exact relation existing between the haemoglobin and 

 the substance of the cell is but little understood. 

 They are but loosely combined, for the haemoglobin 

 is readily dissolved out by water, which itself be- 

 comes colored, while the colorless and swollen cell 

 or stroma is left behind. The shape of haemoglobin 

 crystals obtained from the blood of different ani- 

 mals is not always the same ; those from human 

 blood have, in general, the form of rhombic prisms. 



3. Blood Placques. These are always found in 

 varying numbers in blood when drawn from the ves- 

 sels, and may be seen in the blood during life. They 

 are colorless, oval or round shaped discs of \ to \ 

 the diameter of a red blood-cell. Their significance 

 has not been definitely settled as yet. They are 

 apparently a very important factor in certain patho- 

 logical processes. 



The above general description of the blood-cells 

 of man applies, with few exceptions, to other mam- 

 malia. The differences in size, however, which exist 

 between the red blood-cells of man and those of 



