332 PKACTICAL PHYSIOLOGY 



suspended in an almost colourless fluid, the plasma. In this fluid, too, 

 are certain other corpuscles, which, being colourless, are known as the 

 white corpuscles. When blood is shed, it sets at first to a red jelly. 

 After a time this jelly contracts and gradually squeezes out a pale 

 yellowish fluid, known as the serum. The blood of different species 

 clots at different rates, but the process is essentially the same. 



THE CLOTTING OF BLOOD. 



Demonstration. Inspect the blood clot in the large vessel placed 

 for demonstration purposes. Notice that the shrunken clot floats in 

 the serum. If it be a clot of horse's blood, notice the "buffy coat" at 

 the top, due to the fact that the heavier red corpuscles have had time 

 to sink before the blood clotted, thereby leaving the white corpuscles 

 in abundance at the top. These form "the buffy coat." 



EXPERIMENT I. Carefully sterilise a needle, prick the finger, and 

 draw some blood into a fine capillary tube. Place aside and examine 

 under the microscope at the end of the lesson. 



In order to study the nature of the processes involved in the 

 coagulation of blood, it is essential to stop clotting from taking place. 

 This can be done in several ways, such as by receiving blood into 

 certain neutral salts (J volume of magnesium sulphate, equal volume of 

 sodium sulphate), or into a soluble citrate, oxalate, or fluoride. How 

 these bodies act we shall consider later. Upon standing, the corpuscles 

 will gradually sink, and the supernatant plasma can be pipetted off, or, 

 what is better, the mixture can be centrifugalised and the plasma 

 more quickly obtained. The plasma bears the name indicating its 

 method of preparation; we get therefore "salted plasma," with magnesium 

 sulphate and sodium sulphate, "oxalate plasma" "fluoride plasma" 

 and so on. 



EXPERIMENT II. Of the "salted plasma" provided, take about 

 5 c.c. in three tubes, a, b, c. Dilute each six times with water. Leave a 

 as it is. To b add a few drops of serum. To c add a few drops of 

 serum which has been previously heated for several seconds at 60 C. 

 Place all three in a water bath at 37 C. Note that a and c clot at 

 about the same time, b clots much more quickly. 



From this we learn: (1) that salted plasma clots on dilution; (2) 

 that the process is quickened by the addition of serum ; (3) that the 

 quickening power of the serum is destroyed by previously heating 

 to 60 C. 



EXPERIMENT III. Take about 5 c.c. of oxalate plasma in four 

 tubes, a, b, c, d. Leave a as it is. To b add a few drops of serum. To c 



