ioo AN INTRODUCTION TO ZOOLOGY 



take the form of spots or maculae. Each ampulla possesses a well- 

 marked crista, while the maculae acusticae may occur in the utriculus, 

 sacculus or lagena, in which last part there is a spiral band of 

 sensory cells. Branches of the eighth or auditory nerve are dis- 

 tributed to all these sensory areas. 



The cavity of the middle ear or tympanic cavity is a hollow at the 

 outer side of the auditory capsule, separated from the outside by the 

 tightly stretched tympanum, which is supported by a cartilaginous 

 ring, the annulus tympanicus. To the tympanum is attached a bony 

 rod with cartilaginous ends, the columella auris, which passes in- 

 wards across the roof of the tympanic cavity. The otic capsule is 

 pierced by a small aperture, the fenestra ovalis, which would put the 

 perilymph space in communication with the cavity if it were not 

 closed by a cartilaginous disc, the stapes, on to which the columella 

 fits. Ventrally, the ear cavity is in communication with the back of 

 the buccal cavity, as we have already seen, by means of the Eusta- 

 chian tube. In this way the pressure on the two sides of the tympa- 

 num is kept the same, and so instead of being bulged either in or 

 out with variations in pressure on the two sides it is kept flat and 

 taut. When sound waves strike the tympanum they cause it to 

 vibrate, and this vibration is given to the columella, which in its 

 turn passes it on to the stapes, thence it travels to the perilymph 

 and so finally to the endolymph, which provides the actual stimulus 

 for the sensory cells. 



The function of the ear is twofold. The more obvious is, that it 

 is the organ of hearing, the other function, which although not so 

 obvious is nevertheless the more primitive, is equilibration. The 

 ear of the lower vertebrates is mainly an organ concerned with the 

 balance of the body, and in some cases it is doubtful whether the 

 animals can hear at all, as we understand the term. As we pass up 

 to the higher animals the sense of hearing becomes more and more 

 important, and we lose sight of the primitive function until something 

 occurs that throws it out of gear. The semicircular canals are 

 mainly concerned with equilibration, while the higher developments 

 of the sense of hearing are dependent on the cochlea. 



In the nose, the eye and the ear we have had examples of three 

 sense organs, and in spite of their obvious differences we can find 

 certain underlying features that are common to them all. Each of 

 them is connected to the brain by means of its own special nerve, 

 each is lined by an epithelium containing more or less highly modified 

 sensory cells, and this epithelium, as we shall see later, is derived 

 directly or indirectly from the ectoderm of the embryo. These 

 constitute what are termed the essential parts of the organ, but in 

 addition we always find a number of other structures, the accessory 



