616 THE HUMAN BODY. 



Then stop the tumbler, and the water will go on rotating for 

 some time. Now if the head be turned or rotated in a hori- 

 zontal plane similar phenomena will occur in the endolympli 

 of the horizontal canal ; if it be bent sidewise in the vertical 

 plane, in the anterior vertical canal ; and if nodded, in the 

 posterior vertical; the hairs moving with the canal would 

 meet the more stationary water and be pushed and so, possibly, 

 excite the nerves at the deep ends of the cells which bear 

 them, and generate afferent impulses which will cause the 

 general nerve-centres of bodily equilibration to be differently 

 acted upon in each case. Under ordinary circumstances the 

 results of these impulses do not become prominent in con- 

 sciousness as definite sensations ; out they are probably always 

 present. If one spins round for a time, the endolympli takes 

 up the movement of the canals, as the water in the tumbler 

 does that of the glass ; on stopping, the liquid still goes on 

 moving and stimulates the hairs which are now stationary; 

 and we feel giddy, from the ears telling us we are rotating 

 and the eyes that we are not; hence difficulty in standing 

 erect or walking straight. A common trick illustrates this 

 very well : make a person place his forehead on the handle 

 of an umbrella, the other end of which is on the floor, and 

 then walk three or four times round it, rise, and try to go out 

 of a door ; he will nearly always fail, being unable to combine 

 his muscles properly on account of the conflicting afferent 

 impulses. This and the feeling of rotation in the contrary 

 direction when a previous rotation ceases become readily intel- 

 ligible if we suppose feelings to be excited by relative move- 

 ments of the endolymph and the canals inclosing it. 



The Midbrain. The general arrangement of these parts 

 has been already described (Fig. 82). Cross-sections show 

 (Fig. 176) the aqueduct of Sylvius, 8, traversing the mid- 

 brain near its upper part and surrounded by a thin layer of 

 gray matter, in close connection with which are the origins 

 of the third and fourth cranial nerves, iv, and of part of the 

 fifth. The crura cerebri form the main mass of the midbrain. 

 Each is divided by gray matter (locus niger, Ln) into a ven- 

 tral portion (pes or crusta, P), which forms the semicylin- 

 drical portion of the cms seen on the base of the brain and a 

 dorsal portion, the tegmentum, Tg. The pes consists mainly 

 of the fibres of the pyramidal tract, Py, but some fibres of the 

 fillet, /, also run forward in it, as do fibres, fr and oc, connecting 



