CRANIAL NERVES 205 



second, the time varying with the sensation employed. It is 

 shortened by practice, so that in time it becomes more of the 

 reflex type. 



Medical Significance of Reflexes. Reflexes are separable into 

 superficial and deep reflexes. The former are those which are 

 aroused by stimulation of the surface of the body, i. e., from 

 exteroceptive fields. The latter are aroused from receptors in 

 the depth of the organism, i. e., from proprioceptive fields. 

 Examples of superficial reflexes are the plantar reflex (drawing 

 up of the foot when the sole is tickled), the cremasteric reflex 

 (retraction of the testicle when the skin on the inside of the thigh 

 is stroked), and the gluteal, abdominal, epigastric, and inter- 

 scapular reflexes (contraction of the muscles in those regions 

 when the skin overlying them is tickled). The reflex behavior 

 of the great toe is of diagnostic importance. Normally, on 

 tickling the sole, the toes are flexed toward the planta, but when 

 a lesion of the pyramidal tract exists, as in hemiplegia, there 

 is a dorsal flexion and the toe moves more slowly than normally. 

 This is known as Babinski's sign. In children it is positive 

 during the first few months of life. 



An example of a deep reflex is the knee-jerk. Its absence is 

 of the utmost importance in the detection of tabes dorsalis. 

 When its presence is doubtful, it is necessary to use the reen- 

 forcing effect of simultaneously clasping the hands tightly. 

 While in locomotor ataxia the posterior limb of the reflex arc 

 is deficient, in anterior poliomyelitis the central and efferent 

 portions of the arc are missing. In primary spastic paraplegia, 

 associated with degenerative changes in the lateral columns, 

 the deep reflexes are all exaggerated. 



Cranial Nerves. The cranial nerves, with the exception of the 

 olfactory and optic nerves, arise from gray matter of the medulla 

 and midbrain. The floor of the fourth ventricle is distin- 

 guished particularly by the abundance of nuclei from which 

 the cranial nerves arise. 



I. The olfactory nerve forms the pathway for the impulses 

 giving rise to smell. The fibers pass from the sense endings of 

 the nose to the olfactory bulb on the same side, and then by 

 way of the olfactory tract to the gyrus fornicatus or to the 

 temporal end of the gyrus hippocampi. 



