58 A MANUAL OF PHYSIOLOGY 



it. If it neither rises nor sinks, the specific gravity of the blood 

 is 1*059. If it sinks, the experiment must be repeated with a 

 mixture of higher specific gravity, say 1*061 ; if it rises, with a 

 mixture of lower specific gravity, say i '057, and so on. If the drop of 

 blood rises in mixture i'o6i and sinks in mixture 1-059, the specific 

 gravity is between those two figures, and may be taken as ro6o. 



3. Coagulation of Blood. (i) Take two tumblers or beakers, 

 label them a and ft and measure into each 100 c.c. of water. 

 Mark the level of the water by strips of gummed paper, and pour it 

 out. (If a sufficient number of graduated cylinders is available, they 

 may of course be used, and this measurement avoided.) Into a put 

 25 c.c. of a saturated solution of magnesium sulphate, and into 

 ($ 25 c.c, of a i per cent, solution of potassium oxalate in normal 

 saline solution (*y per cent, solution of sodium chloride). If the 

 dog provided is a large one, these quantities may be all doubled, 



(2) Insert a cannula into the central end of the carotid artery of a 

 dog anaesthetized with morphia* and ether, or A.C.E. mixture.! 



To put a Cannula into an Artery. Select a glass cannula of 

 suitable size, feel for the artery, make an incision in its course 

 through the skin, then isolate about an inch of it with forceps or a 

 blunt needle, carefully clearing away the fascia, or connective tissue. 

 Next pass a small pair of forceps under the artery, and draw two 

 ligatures through below it. If the cannula is to be inserted into the 

 central end of the artery, tie the ligature which is farthest from the 

 heart, and cut one end short. Then between the heart and the other 

 ligature compress the artery with a small clamp (often spoken of as 

 ' bulldog ' forceps). Now lift the artery by the distal ligature, make a 

 transverse slit in it with a pair of fine scissors, insert the cannula, and 

 tie the ligature over its neck. Cut the ends of the ligatures short. 

 If the cannula is to be put into the distal end of the artery, both 

 ligatures must be between the clamp and the heart, and the bulldog 

 must be put on before the first ligature (the one nearest the heart) 

 is tied, so that the piece of bloodvessel between it and the ligature 

 may be full of blood, as this facilitates the opening of the artery. 



(3) Run into a and ft enough blood to fill them to the mark. 



(4) Take a small thin copper or brass vessel, and place it in a 

 freezing mixture of ice and salt. Run into it some of the blood from 

 the artery. It soon freezes to a hard mass. Now take the vessel 

 out of the freezing mixture and allow the blood to thaw. It will be 

 seen that it remains liquid for a short time and then clots. 



(5) Run some of the blood into a porcelain capsule, stirring it 

 vigorously with a glass rod. The fibrin collects on the rod ; the 

 blood is defibrinated and will no longer clot. 



(6) Now let the dog bleed to death, and collect the whole of the 

 blood in a jar. Observe that the flow of blood is temporarily 



* One to two centigrammes of morphia hydrochlorate per kilogramme of 

 body-weight should be injected subcutaneously about half an hour before 

 the operation. Ten c.c. of a 2 per cent, solution is sufficient for a dog ot 

 good size. Note that diarrhoea and salivation are caused by such a dose. 

 For directions for fastening the dog on the holder, see footnote on p. 176. 



f A mixture of equal parts of alcohol, ether, and chloroform. Half of 

 the chloroform in the mixture may be advantageously replaced by gasoline. 



