PRACTICAL EXERCISES 887 



the atlas, and divide the muscles down to the bone just behind these 

 processes. Now start artificial respiration (p. 187), or sooner if 

 necessary. Notch the spinous process of the axis with bone forceps. 

 Pass a strong thick ligature by a sharp-ended aneurism needls close 

 under the body of the axis and tic it tightly in the groove left by the 

 incision behind the transverse processes of the atlas and the notch 

 made in the spinous process of the axis. This compresses the 

 vertebral arteries where they pass from transverse process of axis 

 to transverse process of atlas. Pass a second strong ligature 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 employing the skeletal muscles can be fairly well elicited for 

 hours. Study on the preparation the reflexes described in the text 

 (pp. 799, 80 1) 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. 



