ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



139 



Demonstration of Arterial and Venous Pressure by the Method of 

 Stephen Hales. An incision is made in the mid-line of the neck, 

 from the larynx to the sternum of the anaesthetised cat. The 

 skin-flaps are pulled apart, and the sterno-mastoid and sterno-thyroid 

 muscles separated, so as to expose the carotid artery. With an 

 aneurism needle the artery is freed from the carotid sheath for the 

 space of about an inch. Two ligatures are then placed beneath 

 the artery, and one is tied at the upper end of the exposed portion. 

 On the lower end an artery clip is placed. With sharp scissors an 

 oblique cut is next made into the artery, and the nozzle of the 

 arterial cannula is inserted, and tied in with the second ligature. 

 Lastly the ends of this ligature are brought round the bulb of the 

 cannula, and tied to make the connection secure. 



The arterial cannula is J_ shaped and provided with a bulbous 

 enlargement. This shape is chosen both to hinder clotting and to 



FIG. 136. Arterial cannula. 



facilitate the washing out of clots. One limb of the JL is fitted with a 

 short piece of rubber tube, and this is closed by a piece of glass rod or 

 a clip. The other limb is connected by a short length of thick rubber 

 tube (pressure tubing) to a long length of fine bored glass tubing. The 

 latter must be at least 5 feet in length, and is held in the vertical 

 position by a clamp. The glass tube and cannula are filled with 

 1 per cent, sodium citrate, and this decalcifies the blood and so 

 prevents clotting. The solution is coloured with methylene blue, and 

 a long strip of white paper scaled in centimetres is placed behind 

 the tube. 



By cutting through the attachment of the sterno-mastoid muscle, the 

 junction of the jugular with the subclavian veins is next exposed. The 

 innominate vein is picked up and cleaned with the aneurism needle. 

 Two ligatures are placed under it, and a clip on the part nearest the 



