640 MANUAL OF PHYSIOLOGY. 



which inform the equilibrating centres as to the position of the 

 head. 



By depriving a frog of these several portals by which incoming 

 stimuli direct the balancing centres, it can be rendered incapable 

 of any of the acts requiring equilibration, even when the regu- 

 lating 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 towards any near object, we find 

 how helpless our volition becomes if deprived of the normal in- 

 coming 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, com- 

 posed 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 

 towards 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 



