96o THE CENTRAL NERVOUS SYSTEM 



under the trachea at the level of the cricoid cartilage and include in it 

 the whole neck, except the trachea, but at present only tie a single loop 

 on it. Now decapitate the animal with a large knife (an amputating 

 knife) passed from the ventral aspect of the neck through the occipito- 

 atlantal space, severing the cord just behind its junction with the bulb. 

 At the moment of decapitation tighten the ligature round the neck and 

 complete the knot. Destroy the head. If there is oozing of blood from 

 the vertebral canal, arrest it by raising the neck somewhat above the 

 level of the body. The carcass must be kept warm by placing it on a 

 metal box or table containing hot water, and the air used for artificial 

 respiration must also be warmed, as by passing it through a coil of 

 rubber tubing immersed in a water-bath which is kept hot. Stitch the 

 skin-flaps together so as to cover the cut end of the spinal cord and the 

 other structures cut in decapitation. By this procedure the spinal cord 

 is usually severed about 4 millimetres behind the point of the calamus 

 scriptorius. Although the blood-pressure remains low, reflexes employ- 

 ing the skeletal musles can be fairly well elicited for hours. Study on 

 the preparation the reflexes described in the text (pp. 873, 876) e.g., the 

 flexion reflex of the hind and fore limb, as elicited from the skin, or one 

 of the afferent nerves of the limb the crossed extension reflex of hind 

 and fore limb, the scratch reflex. 



(1) Scratch Reflex. (a) Evoke the reflex by rubbing the skin of the 

 neck behind the pinna. The scratching movements are in the hind-leg 

 of the same side. Record them on a drum, on which is also written a 

 time-tracing in seconds. The record can be obtained by tying a piece 

 of tape, not too tightly, round the foot, leg, or thigh, and connecting 

 this by a thread with a lever. The thread is passed over a pulley below 

 the lever, so that its pull may be exerted at right angles to the axis of 

 rotation of the lever. The lever is attached to a light spring or a rubber 

 band, which is stretched when it moves in one direction, and in recoiling 

 brings it back again to its position of rest at the end of the contraction. 

 If the reflex is not easily evoked, it can be facilitated by producing a 

 slight degree of asphyxia by temporarily clamping the respiration tube. 

 Some time must elapse after the decapitation before a fair scratch reflex 

 can be expected. It is usually sufficiently well marked within an hour. 



(b) While the reflex is occurring, stimulate with an interrupted current 

 the central stump of the popliteal nerve of the opposite hind-limb. 

 The scratch reflex may be cut short by inhibition. Also, during the 

 stimulation of this nerve the reflex may be incapable of being elicited 

 till the excitation of the inhibitory afferent nerve is stopped. 



(2) Flexion Reflex. (a) Stimulate with a weak interrupted current 

 the skin of some part of the hind-limb say one of the toes. The flexion 

 reflex of the hind-limb on the same side may be evoked i.e., a flexion 

 movement at the knee, hip, and ankle. Record the movements of one 

 of the joints or of flexor muscles after severing them from their insertion. 



(b) Stimulate, with a weak interrupted (faradic) current the central 

 stump of one of the nerves of a hind-limb say the peroneal nerve. 

 The flexion reflex of the same limb may be elicited. Record the move- 

 ments. Now produce temporary asphyxia by clamping the respiration 

 tube, and repeat the stimulation at half -minute intervals. The reflex 

 will be increased by the asphyxia. Do not interrupt the respiration for 

 more than two or three minutes, and immediately start it if the heart, 

 which can be felt through the chest, begins to weaken. 



(3) Elicit the knee-jerk, as described in the text (p. 876). It is 

 generally exaggerated. 



(4) By the unipolar method (p. 918) stimulate with a point electrode 

 one lateral half of the cross -section of the cervical cord exposed in 



