CRURA CEREBRI. 639 



impulses which inform the equilibrating centres as to the position 

 of the head. 



By depriving a frog of these several portals by which incom- 

 ing stimuli direct the balancing centres, it can be rendered in- 

 capable of any of the acts requiring equilibration, even when 

 the regulating centres are intact. In our own bodies we can 

 convince ourselves of the absolute importance of these afferent 

 regulating impulses arriving from the ear, eye, skin, and muscles. 

 If having bent one's forehead to the handle of a walking stick, 

 the end of which is fixed on the ground, we run three or four 

 times around this axis, and then quickly walk toward any near 

 object, we find how helpless our volition becomes if deprived of 

 the normal incoming stimulus, for thus an unwonted disturbance 

 of the nerve terminals in the semicircular canals has dispatched 

 conflicting impulses to the coordinating centre of equilibration. 

 Further, we know that we stand less fixedly when our eyes are 

 shut, and we move unsteadily when our feet are benumbed, etc. 



CRURA CEREBRI. 



Passing above the Pons Varolii, we come to a thin isthmus, 

 composed of two thick strands of nerve substance connecting the 

 mesencephalon with the cerebral hemispheres. These are called 

 the crura cerebri. They diverge slightly in their upward course 

 toward the hemispheres, and lie just below the corpora quadri- 

 gemina, which have already been referred to. Minute examina- 

 tion of these crura brings to light an anatomical difference which 

 corresponds with a distinct physiological separation between the 

 paths taken by the sensory and motor impulses in each crus. 

 The lower or more anterior part, which can be seen on the base 

 of the brain, is called the base or crusta. This is made up of 

 motor nerve channels only. The posterior or upper part, which 

 lies next to and is connected with the corpora quadrigemina, is 

 called the tegmentum. and is composed of sensory fibres. Ana- 

 tomically, the separation between the two is indicated by some 

 scattered nerve cells (locus niger). The base, or crusta, which is 

 the great bond of union between the spinal cord and the cerebral 

 motor centres, passes into the corpus striatum ; and the tegmen- 



