THE SENSE-ORGANS 395 



In birds and mammals there is no pineal foramen in the 

 skull, and the pineal organ remains beneath the bone. It is 

 reduced to a solid vestige and its function is changed from that 

 of a visual organ to an organ of internal secretion, or ductless 

 gland. 



The Ear. — The most primitive part of the ear is the 

 utricular portion with its semicircular canals and ampullae. 

 Myxine has one, and Petromyzon has two semicircular canals 

 on each side. All other Craniates have three, in planes at 

 right angles to each other. In the ampullae are the statolithic 

 particles which are supported on sensory cilia. Gravity makes 

 these particles weigh on the cilia immediately beneath them, 

 whatever the position of the animal, and so the animal is 

 informed of its position with regard to the vertical according 

 as to which of the cilia are so stimulated. The semicircular 

 canals contain fluid, the endolymph, as do all parts of the 

 auditory sac. When the animal starts or ceases moving, a 

 flow of endolymph takes place in the semicircular canals, 

 which resolve the direction of the movement into resultants 

 in the three planes of space in which they lie. While the 

 statoliths are static, the semicircular canals are dynamic organs 

 of balance. 



Hearing is the perception of mechanical vibrations of low 

 frequency. Fish are capable of hearing with their auditory 

 organs, but this sense only becomes important in the verte- 

 brates which have left the water, and are therefore subject to 

 vibrations in air. This is significant because these animals 

 are also the first to emit vocal sounds. Since these animals 

 are autostylic and no longer breathe by gills, the spiracular 

 cleft and the hyomandibula are no longer needed to subserve 

 their primitive functions ; they give rise to the tympanic 

 cavity (and Eustachian tube) and columella auris (stapes) 

 respectively. The vibrations of air impinge on the tympanic 

 membrane or ear-drum, and are conveyed by the columella 

 auris across the tympanic cavity to the auditory capsule. The 

 wall of the auditory capsule is imperforate in the fish and 

 in the most primitive Stegocephalia (Eogyrinus). In the 

 remaining vertebrates the auditory capsule has two openings 



