250 CIRCULATION OF THE BLOOD. 



the crown of the arch ; here they descend parallel to each other, 

 but not in contact. Below the arch is a flat vulcanite bag, on 

 the upper surface of which a U tube is supported vertically, 

 with its concavity upwards. The ends of the two wires are 

 received into the two limbs of the U. As the bend contains 

 mercury, it is obvious that whenever the bag expands tlie cir- 

 cuit is closed, and broken when it contracts. The rest of the 

 mechanism is so arranged that the tube is closed beyond the 

 breaker whenever the magnet is not acting, and open so long 

 as the current passes. This condition can, however, never be 

 permanent ; for after an interval of time, which can be very 

 readily regulated by altering the quantity of mercury in the U 

 tube, the bag becomes sufficiently distended to close the cir- 

 cuit. When this happens, the magnet acts and opens the tube, 

 allowing the distended bag to discharge itself. This contriv- 

 ance answers particularly well for the artificial respiration of 

 rabbits. The needles for exciting the cord are constructed in 

 the same manner as those described in the preceding paragraph ; 

 they should, however, be thicker and stronger. 



The canulfle having been placed in the trachea and external 

 jugular vein, and the apparatus for artificial respiration being 

 in order, three-tenths of a centimetre of a one percent, solution 

 of curare is injected. As soon as respiration ceases, air is in- 

 jected at regular intervals by the metronome, the beats of which 

 express the previous frequency of breathing. The carotid 

 artery is now connected with the kymograph, and the animal 

 placed in the supine position, the head-holder being so arranged 

 that the head is very much flexed on the cervical part of the 

 spinal column, so as to make the space between the occipital 

 bone and the atlas as wide as possible. In doing this, great 

 care must be taken not to strain or twist the artery, or kink the 

 air tube. This done, an observation must be made of the arte- 

 rial pressure, and the atlanto-occipital membrane exposed with 

 as much dispatch and as little bleeding as practicable. This 

 is best effected with the aid of the notched needle, fig. 203 /. 

 With the help of this needle, three ligatures are passed under- 

 neath the muscles which stretch vertically on either side of the 

 spine of the atlas, its point being directed towards the occipital 

 spine as close to the bone as possible. It is usually necessary 

 to pass two such ligatures in line on either side, the upper 

 entering where the lower passes out. The ligatures having 

 been tightened and the muscles divided in the middle line, it 

 is easy to expose the posterior tubercle of the atlas, the mem- 

 brane, and the edge of the occipital bone, without hemorrhage. 



The next step is to expose the cord by dividing the atlanto- 

 occipital membrane; this is best done with scissors and for- 

 ceps. While a tracing of the arterial pressure is taken by an 

 assistant, the cord is divided: at once the mercurial column 



