100 



EXERCISE XVI 



Obs. 77. 

 Head-shake 

 reflex of audi- 

 tory meatus. 



Obs. 76 b. 

 The * law ' of 

 the roots and 

 the pinna 

 reflex. 



scissors. (Note that the cutting of the root probably excites the reflex of the 

 pinna during its performance.) Then re-stimulate the nerve-trunk by ligating 

 as before. No reflex is evoked. 



IV. iv. Cut the nerve distal to the ligatures and free it up to the 

 ganglion ; raising the stump by the ligature, faradize it with unipolar electrode, 

 using a weak stimulus. No reflex results, unless there be escape of current, 

 and this, the nerve being short, is not unlikely to take place unless the current is 

 very weak, and it may then be too weak to excite the reflex. The observation 

 with faradism is therefore less reliable than with the mechanical stimulus. 



IV. v. Eepeat the observation with the nerve of the right eide, but 

 with the difference that the ventral root is cut (PI. VIII, fig. 4) instead of the 

 dorsal. Severance of the ventral root does not annul or in any way affect the 

 reflex. 



V. Free the occipital stump and neck of the preparation from all clip- 

 weights ; support the stump in prone position lightly by a finger under the 

 mandibular symphysis so that neck may be free to move. Blow through 

 a small tube into the concha ; a rapid rotatory shake of the occipital stump 

 will ensue. Repeat by squirting a few drops of water into the concha. Note 

 that the ensuing rotation of the neck is clockwise as viewed from in front 

 of the preparation when the concha stimulated is the left, anti-clockwise when 

 the concha is the right. This is a protective reflex preventing entrance of 

 noxa into the auditory meatus. The chief afferent nerve is auricular branch 

 of vagus. 



[VI. A more complete but lengthier form of Obs. 76 employs the great 

 auricular nerve (see schema, text-fig. 36) in its course behind the ear as the 

 afferent nerve for excitation instead of the 2nd cervical nerve-trunk near the axis 

 vertebra. The procedure then is as follows : 



VI. i. Operation for great auricular nerve. (PI. VIII, figs. 5, 6.) Lay the 

 preparation on its side — on right side preferably, as the accompanying diagrams 

 show operation for left side. 



Skin incision from a point on the posterior face of the concha of the pinna, 

 about 2 cm. behind the free lateral edge of pinna and about half-way between 

 the lower end of that free edge and the lower end of the little lateral skin- 

 pocket, the bursella, on that edge. From the above starting-point the skin 

 incision is carried backwards along the side of the neck for about 5 cm., 

 tending slightly toward the dorsum. From anterior end of it an incision at 

 right angles to it is carried for 2 cm. dorsally : from the posterior incision a 

 similar one for 2 cm. ventrally. Reflect the skin-flaps and retract with clip- 

 weights. 



