A STUDY OF THE SENSES 311 



walled circular membrane which helps to form the partition 

 between the middle ear and the inner ear. Inside of this 

 membrane are two small sacs, the u-tric'u-lus (Latin utric- 

 ulus = a little skin bottle), and a still smaller, sac'cu-lus 

 (Latin sacculus = a small sac). From the utriculus run 

 off three delicate semicircular canals. When we are stand- 

 ing erect, one of these canals in each ear lies in a hori- 

 zontal position, the second is vertical and runs anteriorly 

 and posteriorly, while the third, also vertical, extends from 

 side to side. At one end of each canal is a little swelling, 

 to which runs a branch of the auditory nerve, and other 

 branches supply portions of the walls of utriculus and 

 sacculus. The cells with which these fibers connect are 

 long and slender, and from each projects into the endo- 

 lymph a fine hair. Hence, these cells resemble some- 

 what the sensory cells which lie within the taste buds (see 

 p. 295). 



If I were sitting on the deck of a rolling ship, I could 

 tell, even with my eyes closed, in what direction I was 

 being rocked. We become more or less conscious, too, of 

 the ordinary movements of the head without the use of the 

 eyes, and the impulses that give us these sensations prob- 

 ably come from the semicircular canals in the following 

 way. All these canals and their enlargements, together 

 with the utriculus, are surrounded by perilymph, and, as 

 we have said, they are filled with endolymph. When the 

 head is moved in any direction, for instance, in walking, 

 the liquid endolymph flows against the projecting hairs of 

 the sensory cells, and an impulse is thus started along the 

 auditory nerve. When this reaches the cells of the fore- 

 brain, we become conscious that some change in the posi- 

 tion of the body is taking place. After we have learned to 

 walk, however, we balance our body without any conscious 

 thought, and walking becomes automatic. We may say, 

 then, that the part of the ear we have been describing gives 

 us a knowledge of our position and movements in space. 



