THE BLOOD. 



239 



This group is separated from 



sucked into the tube up to the mark lo'i . On shaking the pipette for a few minutes, 

 the admixture will take place, aided by the movements of the glass ball. 



Fig. 101 shows both a surface view, a, and a section view, b, of the counting 

 chamber. This consists of an oblong glass plate, o, on which are cemented two 

 small pieces of glass, one of which, WD, has in the center a circular opening 

 in which is placed the other, B, a circular disc or stage. Their relation is such 

 that a narrow groove or moat separates the one from the other, the floor of which 

 is formed by the glass plate. The surface of the circular stage is exactly o.i mm. 

 lower than that of the cover-glass, r. On the surface of the glass stage a series 

 of small squares is engraved, C, each one of which has a side length of ^ mm. 

 and an area of -%fa square mm. To facilitate counting, a group of 16 such 

 squares is surrounded by a thick line. Fig. 102. 

 adjoining groups, also enclosed by thick lines, by 

 an intermediate fine line, which serves as a guide 

 in passing from one group to another. When a 

 cover-glass is accurately applied to the glass, b, 

 each one of the small squares will have a cubic 

 capacity of ^Xo.i, or T oVo cubic millimeter. 



Before placing the diluted blood on the count- 

 ing stage, the fluid in the tube of the pipette 

 should be blown out and discarded, as it contains 

 no portion of the blood. A small drop is then 

 placed on the glass stage and covered with the cover- 

 glass. After a few minutes the corpuscles settle 



upon the ruled spaces and are ready for counting. FIG. 102. MICROSCOPIC AP- 

 The number of corpuscles in at least five series PEARANCE OF THE SMALL SQUARES 

 of sixteen small squares is then counted; this AND THE ^DISTRIBUTION OF THE 

 number is then multiplied by the degree of dilu- 

 tion (100 or 200 as the case may be) and this divided by the cubic contents 

 of each small square (^oVo) '> tne product is then divided by the number of squares 

 counted (So in the instance given above) : e.g., five series of sixteen small squares 

 contain 500 corpuscles 



0^x^X4000 ^ 5)000j00 o erythrocytes per c.mm., 



The accuracy of the counting is proportional to the number of squares counted. 

 If 200 squares are counted with each of two different drops, and the average taken 

 the probable limit of error will be less than 2 per cent. 



The Preservation of the Red Corpuscles in the Plasma. Within the 



blood-vessels the physical conditions and chemic composition of the plasma 

 are such that both the form, and the composition of the corpuscle or the rela- 

 tion of the hemoglobin to the stroma, are maintained in the normal or physio- 

 logic condition. The plasma is preservative of the structure and function 

 of the corpuscle. The reason assigned for this is that the osmotic pressure 

 of the salts in the plasma and of the salts in the corpuscle exactly balance 

 one another so that there is neither an absorption of water from, nor a yield- 

 ing of water to, the plasma on the part of the corpuscle. The plasma having 

 an osmotic pressure equal to that within the corpuscle is said to be isotonic 

 with it. 



When blood is to be prepared for microscopic examination with a view of 

 determining the histologic features of the corpuscles or for purposes of 

 enumeration, it must be diluted, and unless special precautions are 

 observed the condition of equal osmotic pressure will be disturbed by the 



