gg8 THE CENTRAL NERVOUS SYSTEM 



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. 904). It is 

 generally exaggerated. 



(4) By the unipolar method (p. 951) stimulate with a point electrode 

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

 decapitation. The large electrode is placed on a shaved part of a fore- 

 arm. Various effects may be elicited according to the point of the 

 cross-section stimulated e.g., stepping and scratch movements of the 

 hind-limbs. Other facts mentioned in the text in regard to spinal 

 reflexes can be verified on this preparation. 



9. Decerebrate Cat Preparation (Miller and Sherrington) . This 

 preparation, which must be made by the demonstrator, differs from 

 the spinal preparation described in 8, in that the plane of the section 

 is considerably higher, passing through the posterior part of the mid- 

 brain ' entering the posterior colliculi (posterior corpora quadrigemina) 

 near their hinder end and emerging about a millimetre anterior to the 

 front edge of the pons.' Many reactions can be conveniently studied 

 on this preparation, some of which cannot be obtained with the spinal 

 preparation. The respiratory movements usually go on without inter- 

 ruption, and reflexes whose centres are situated in the bulb, such as 

 the swallowing reflex, can be elicited. The circulation is well main- 

 tained, and the preparation can be used for a number of the experi- 

 ments given after Chapters III. and IV. 



Under deep anaesthesia (chloroform and ether), the carotids are 

 temporarily clamped opposite the uppermost tracheal cartilage, and 

 the cat is then placed on the decerebrator (Fig. 401) in the prone posi- 

 tion, with its neck on the upper edge of the neck-block. 



The interparietal suture is exposed by an incision through the scalp 

 from beyond the coronal in front to the lambdoid ridge and supraocci- 

 pital protuberance behind. A distance of 30 millimetres is then 

 measured off from the coronal suture backward along the interparietal 

 and the point is marked by notching the longitudinal median ridge of 

 bone. The head is placed between the prongs of the yoke with the 

 tongue-guard of the yoke-plate inside the mouth above the tongue, 

 which it protects. The chin lies in the hollow of the yoke under the 

 yoke-plate, the lower canines covered by the plate. The pelvis lies 

 on the pelvic platform, P, the hind limbs hanging freely. The head 

 is pushed down, the tongue guard in the mouth, until the embayed end 

 of the yoke-plate on either side of the tongue-guard meets the anterior 

 edge of the coronoid process of the lower jaw. The hook attached 

 to the leather cord is fixed in a loop of string previously tied trans- 

 versely through the upper lip, and the snout is drawn firmly down by 

 the cord, securing the head in position. The cord is fastened to a 

 elect on the under surface of the neck-block. The nose-piece is now 

 slid up so as to engage and support in its notch the apex of the muzzle, 

 and is fixed by the screw-clamp. A knife consisting of a planing-blade 

 mounted in a wooden handle is used for the de cerebration.* 



* The blade is 12-5 centimetres wide, 9 centimetres high, and 4 millimetres 

 thick. It is bevelled on one face, the bevelled edge being 2 centimetres deep. 

 The cutting edge does not extend the whole width of the blade, but stops short 

 at 1*5 centimetres from each lateral edge. The mid-width of the blade is 

 marked by an engraved line on the front of the knife. 



