THE BLOOD. 207 



THE CONNECTIVE TISSUES. 



Three principal modifications or varieties of connective tissue have long been 

 recognized and separately described, viz., areolar tissue, fibrous tissue and elastic 

 tissue. Others, however, belong unmistakeably to the same group, as the study of 

 their structure and the history of their development in the mesoblast clearly show. 

 Of these the most important are adipose tissue, retiform and tymphoid tissue, cartilage, 

 lone and the elements of the Hood and lymph. It will be convenient to study the 

 last-named elements before the structure of the connective tissues proper is considered. 



THE BLOOD. 



The most striking external character of the blood is its well-known colour, which 

 is bright red approaching to scarlet in the arteries, but of a dark purple or modena 

 tint in the veins. It is a somewhat clammy and consistent liquid, a little heavier 

 than water, its specific gravity being about T055 ; ib has a saltish taste, a slight 

 alkaline reaction, and a peculiar faint odour. 



To the naked eye the blood appears opaque and homogeneous ; but, when 

 examined with the microscope, either while within the minute vessels, or when spread 

 out into a thin layer upon a piece of glass, it is seen to consist of a transparent 

 colourless fluid, named the "lymph of the blood," "liquor sanguinis," or "plasma," 

 and minute solid particles or corpuscles immersed in it. These corpuscles are of two 

 kinds, the coloured and the colourless : the former are by far the more abundant, 

 and have been long known as " the red particles," or " globules," of the blood ; the 

 " colourless," " white," or " pale corpuscles," on the other hand, being fewer in 

 number and less conspicuous, were later in being generally recognised. 



"When blood is drawn from the vessels, the liquor sanguinis separates into two 

 parts ; into fibrin, which becomes solid and takes the form of fine interlacing 

 filaments, and a pale yellowish liquid named serum. 



In a cubic millimeter of healthy human blood there are on an average 5,000,000 

 red corpuscles (Vierordt) and 10,000 white corpuscles. The number of white 

 corpuscles varies much more than that of the red, and the proportion of the white to 

 the red is variously given at from 1 : 1000 to 1 :250. There are said to be fewer 

 red corpuscles in the female (4,500,000 in a cubic millimeter 'according to Welcker). 



The numeration of the blood-corpuscles is readily performed. A little blood, obtained by 

 pricking the finger, is measured in a capillary tube, and is then mixed with a measured amount 

 (say 100 times its volume) of dilute solution of sulphate of soda, or some other salt which will 

 maintain its fluidity and at the same time preserve the corpuscles nearly unaltered ; the latter 

 can then be counted in a small known quantity of the mixture. This part of the operation is 

 effected by placing a drop of the mixture in the middle of a glass " cell " of a certain depth 

 (say ith of a millimeter), the bottom of which is ruled in squares, the sides of which are of a 

 known dimension (say again i mill.). If now a covering glass is placed over the cell so as to 

 touch the drop, the latter will form a layer of the mixture ^ mill, deep, and the part above 

 each square will represent a cube of liquid the sides of which measure J g mill. So that by 

 counting the number of corpuscles in ten squares, after allowing them time to subside, and 

 multiplying the result by 10,000, the number in a cubic millimeter of the blood is obtained. 



The methods of Hay em and Nachet, Gowers, and Thoma are based on the above principle. 

 The average results obtained by recent investigators agree closely with the original estimates 

 of Vierordt and Welcker. 



