iooo THE CENTRAL NERVOUS SYSTEM 



The preparation is then removed from the apparatus, the neck being 

 held laterally behind the wings of the atlas to control the vertebral 

 arteries. It is placed on the experiment table with the neck well raised 

 by a string passed through the skin over the occiput, to restrain any 

 haemorrhage. Some cotton is packed across the cut surface of the mid- 

 brain. Bleeding soon ceases, and in two or three minutes one carotid 

 can be undamped, arteries in the masseteric regions being tied if neces- 

 sary. In two minutes more the clamp can be removed from the other 

 carotid. 



10. With the preparation described in 9, study the swallowing reflex, 

 evoked e.g., by the application of water by drops to the pre-epiglotti- 

 dean sinus between the base of the tongue and the epiglottis, or by 

 allowing water to drop into the pharynx. Dilute alcohol (one part of 

 ethyl alcohol added to four parts of water) is even more effective 

 than water; oil much less effective. 



n. Reflex Postural Tonus (Decerebrate Rigidity). Study the dis- 

 tribution of the tonus in the decerebrate cat prepared as in 9 e.g., 

 in the extensor muscle of the knee (the vasto-crureus), the gastrocne- 

 mius, semimembranosus, triceps, supraspinatus, etc. Isolate the knee 

 extensor by paralyzing all the other muscles of both hind limbs by nerve 

 section. The vasto-crureus still maintains its postural reflex contrac- 

 tion. To observe the right vasto-crureus place the animal on its left 

 side. Begin with the knee nearly at full extension and determine what 

 weight, attached to the tibia and pulling it backward by a cord fastened 

 over a pulley i.e., tending to flex the knee is just counteracted by 

 the postural action of the muscle. Now forcibly flex the knee nearly 

 to the full, bending it steadily and not too quickly, so that the move- 

 ment occupies a couple of seconds. Apart from a slight partial return 

 towards extension, and this not always, the limb remains in the new 

 position. Although the length of the vasto-crureus is now greater 

 than before, the weight needed to counterbalance its pull is practically 

 the same. That is to say, the muscle has assumed a new postural 

 length without any sensible change of tension. This is the so-called 

 ' lengthening reaction ' of the posturally contracted muscle. The 

 ' shortening reaction ' can be obtained by repeating the observations 

 in the reverse order i.e., starting with the knee in nearly full flexion 

 (Sherrington) . 



12. Reflexes in Man. Study systematically on a fellow-student 

 and on yourself the chief reflexes described in the text (p. 914), 

 especially 



The Knee-jerk. (i) Elicit the jerk in the usual way by striking the 

 ligamentum patellae and observe its height. Then cause the patient to 

 make a strong voluntary movement (squeezing the hands together or 

 clenching the jaws) at the moment when the tendon is struck, and note 

 whether the height is increased by ' reinforcement.' 



(2) Attach a suitable recording apparatus to the foot of a person 

 sitting with his legs over the edge of a table, and record the jerks 

 elicited by taps made as uniform in strength as possible. A small 

 hammer worked by an electro-magnet or a spring might be employed 

 for this purpose. Compare the records obtained when the jerk is 

 elicited while the person is squeezing his hands together with those 

 previously obtained. The influence of mental activity, especially of ex- 

 citement or irritation (opportunities of studying such physical states 

 occasionally offer themselves in physiological laboratories) in increasing 

 the height of the knee-jerk may also be verified (Lombard). 



13. Excision of Cerebral Hemispheres in the Frog (Fig. 402). 

 Anaesthetize a frog lightly by putting it under a bell-jar or tumbler 



