SEC. 5. THE MACHINERY OF COORDINATED 



MOVEMENTS. 



642. We may now direct our attention for a while to some 

 considerations concerning the nature of this complex nervous 

 machinery for the coordination of bodily movements, and espe- 

 cially concerning the part played by afferent impulses. Most of 

 our knowledge on this point has been gained by a study of animals 

 not deprived of, but still possessing their cerebral hemispheres, or 

 by deductions from the data of our own experience ; but it is 

 possible in most cases to eliminate from the total results the 

 phenomena which are due to the working of a conscious intelli- 

 gence. Some of the most striking facts bearing on this matter 

 have been gained by studying the effects of operative interference 

 with certain parts of the internal ear, known as the semicircular 

 canals. The details of the structure of these parts we shall 

 describe later on when we come to deal with hearing, but we 

 may here say that each internal ear possesses three membranous 

 semicircular canals, disposed in the three planes of space (one 

 horizontal, and one in each of the two vertical planes, fore and 

 aft and side to side), each membranous canal being surrounded 

 by a bony canal of nearly the same shape, and being expanded 

 at one end into what is called an ampulla, on which fibres of 

 the auditory nerve end. Each membranous canal, in common 

 with the cavity of the internal ear of which it is a prolongation, 

 contains a fluid allied to lymph, called endolymph, and the space 

 between each membranous canal and its corresponding bony canal 

 is in reality a lymph space, containing a fluid which is virtually 

 lymph, though it is called by the special name of perilymph. 

 In birds interference with the semicircular canals produces the 

 following remarkable results. 



When in a pigeon the horizontal membranous semicircular 

 canal is cut through, the bird is observed to be continually moving 

 its head from side to side. If one of the vertical canals be cut 

 through, the movements are up and down. The peculiar move- 

 ments may not be witnessed when the bird is perfectly quiet, but 



642 



