220 SPECIAL PHYSIOL OGY 



(13) Draw a map of the tongue, locating those areas most sensitive 

 to the four tastes, severally. 



/. Auditory Sensation. (14) To test acuteness of hearing, deter- 

 mine how far the subject can hear a watch tick when the timepiece 

 is held at the level of the head, and some distance to one side. Record 

 distance in centimetres. 



(15) To determine the highest pitch discernible by the ear test the 

 subject with a Galton whistle, recording the number of vibrations per 

 second audible. 



g. Visual Sensation. (See Chapter on Vision.) 



VIII. FUNCTION OF SPINAL NERVES. 



It is intended here simply to outline a technique which may be 

 used in making tests of any efferent nerve trunk. 



In testing a spinal nerve trunk one has the choice of stimulating 

 the anterior root the efferent or motor root or of stimulating the 

 whole trunk. If one stimulates the anterior root only there is no 

 need of cutting the nerve root next to the cord, as all impulses are 

 efferent. 



t In testing a nerve trunk, it is, however, necessary to cut the nerve 

 and stimulate the distal end only, if one wishes to observe the action 

 of the motor fibres. If the trunk were not cut, some of the fibres, 

 being afferent and sensory, would carry impressions to the cord and 

 elicit a reflex response which would seriously complicate the obser- 

 vation of the direct efferent impulses from the point of stimulation 

 to the motor distribution of the nerve. Results to be of any value, 

 therefore, must be gotten through the electric stimulation of distal 

 ends of cut nerve trunks. 



1. Appliances. Dry cell; inductorium; contact key; short-circuit- 

 ing key; three wires; shielded electrodes with wires; small dog or 

 large rabbit; chloroform or ether; clippers; operating case. 



2. Preparation. Fasten animal to holder, anaesthetize, clip the 

 throat, and set up electric apparatus for single-induction shocks. 



3. Operation. Dissect out any nerve trunk which it is proposed 

 to test. Take, for example, one of the several trunks of the brachial 

 plexus; take the fifth cervical. 



Make a cutaneous incision along the outer margin of the sterno- 

 cleidomastoid muscle. Separate the subcutaneous tissues and deeper 

 tissues to expose the cervical nerve trunks as they emerge from between 

 the scalene muscles. Identify the fifth cervical trunk and separate 

 it from the surrounding tissues sufficiently to permit the introduc- 

 tion of the shielded electrode beneath it. Tie a ligature around 

 the nerve close to the spinal column and cut the nerve beyond the 

 ligature. 



