THE BRAIN. 493 



and also instances in the human subject in which during life 

 the movements had been co-ordinated, yet after death the cere- 

 bellum had been found completely disorganized. 



It is interesting to know that in animals that produce complex 

 movements the cerebellum is considerably developed, while in 

 those whose movements are simple, such as the frog, this organ 

 is exceedingly small. 



Sources of Impressions that reach the Cerebellum. The ana- 

 tomic relations of the cerebellum are so intimate that impressions 

 of many kinds can reach it and thus enable it to preside over this 

 most important function of co-ordination, especially as regards 

 equilibration and locomotion. 



Equilibration. Sewall defines equilibrium as a a state in which 

 all the skeletal muscles are under control of nerve-centers, so that 

 they combine, when required, to resist the effect of gravity or to 

 execute some co-ordinated motion." In order that these centers 

 may send to the muscles of the body impulses that are adapted 

 to produce the desired result, it is absolutely essential that they 

 should receive impressions which will give them cognizance of the 

 exact position of the body and of the condition of the muscles at 

 the moment as to contraction or relaxation. These impressions or 

 sensations taken as a whole constitute the sense of equilibrium, 

 and while it is doubtless true that all sensations contribute to 

 bring about this result, yet there are some which are more directly 

 concerned than others. These are impulses received from the 

 skin, from the muscles, and from the semicircular canals, the last 

 being doubtless the most important. 



Impressions from the Semicircular Canals. These are some- 

 times described under the name labyrinthine impressions. For a 

 description of the labyrinth, the reader is referred to page 607. 



Although in intimate anatomic relationship with the organs of 

 hearing, there is still no doubt that the semicircular canals bear 

 no physiologic relationship with that special sense, for removal of 

 these structures leaves hearing unimpaired, provided, of course, 

 the cochlea are uninjured. On the other hand, serious disturbances 

 of equilibrium do result, and these vary according as one or 

 another of the canals is destroyed. Thus, if in a pigeon the 

 horizontal canal is destroyed, the bird moves its head from side to 

 side around a vertical axis : whereas if the injury is to the 

 superior canal, the movements are vertical around a horizontal 

 axis. When a pigeon whose semicircular canals have been injured 

 is in a resting posture, it stands with its head turned backward or 

 forward or to one side, never in a natural position ; and when dis- 

 turbed, its movements are irregular, accompanied by rolling of the 

 eyes and an inability to fly. If the injury is limited to one side, 

 recovery soon takes place ; while if the canals on both sides are 

 destroyed, the condition is a more persistent one. Different ani- 

 mals act somewhat differently after injury to the canals ; in mam- 



