INTRODUCTION OF CANNULA. 259 



diminished for thermal irritations, but not for tactile ones — 

 pinching producing reflex action, but burning or pricking none. 

 It leaves the motor nerves, vagus, and muscles unaffected ; but 

 diminishes the activity of the respiratory nervous centre, 

 rendering the breathing slow, and of the cardiac ganglia, 

 somewhat weakening the heart. It lessens the blood-pressure 

 and temperature, probably by dilating the vessels at the surface, 

 as the ear of a rabbit becomes hot and its vessels dilated, while 

 the general temperature is falling. 



Opium is a mixture of several alkaloids, some of which are 

 purely narcotic, while others produce tetanic spasms just like 

 strychnia, and others partake of both characters. This is the 

 case with morphia, in which, however, the narcotic qualities 

 predominate. In small doses it tirst slightly increases, and 

 then diminishes the irritability of motor and sensory nerves, 

 the reflex action of the cord, the irritability of the vagus (ends 

 and central roots), the musculo-motor apparatus of the heart, 

 and the temperature. If the dose be large, those functions 

 may be at once lessened. The blood-pressure varies, but is 

 generally raised. 



The advantage of giving either a narcotic or the drug to be 

 investigated by injection into a vein rather than subcutaneously 

 is, that the nction is immediate, and we know that the whole of 

 the dose has taken effect; whereas, after subcutaneous injec- 

 tion, a part may remain for some time in the cellular tissue 

 before it enters the blood and becomes active. The most con- 

 venient vein is the external jugular. In dogs, however, it is 

 sometimes more convenient to inject the narcotic into a vein 

 which runs obliquely across the outside of the hind knee-joint. 

 Before injecting, we must introduce a cannula into the vein ; 

 and the introduction of a cannula into a vessel is an operation 

 on the proper performance of which the success of many an 

 experiment depends. 



Introduction of Cannulce into Vessels. — First the hair must 

 be cleanly clipped or shaved away, and loose hairs removed by 

 a moist sponge. The skin, subcutaneous cellular tissue, and 

 cutaneous muscles, are divided with a scalpel, and any bleeding 

 vessels are twisted or ligatured. If the vessel lie deep, the 



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