XVII THE SENSE ORGANS 335 



the sensory apparatus ; the other, the middle ear, being com- 

 posed of accessory structures for the transmission of sound 

 waves to the sensitive part of the organ. 



The inner ear is composed of the membranous labyrinth, 

 which hes within the auditory capsule, which is formed by 

 the prootic and exoccipital bones of the skull. The laby- 

 rinth is a complicated sac formed originally by invagination 

 of the ectoderm of the surface of the body from which it is 

 subsequently constricted off. It is divisible into an upper 

 portion, the ntriculus, and a lower, smaller part, the saccu- 

 lus. The former is an oblong sac lying nearly horizontal. 

 It gives rise to the three semicircular canals, which lie in 

 planes approximately at right angles to each other. These 

 canals are membranous tubes embedded within the cartilage 

 of the auditory capsule and communicating with the utriculus 

 at each end. There is an anterior canal which lies in a 

 vertical plane, a lateral canal which lies in a nearly horizon- 

 tal plane on the outer side of the utriculus, and a posterior 

 canal which extends transversely nearly at right angles to the 

 two others. At one end each of the canals is furnished with 

 an enlargement, or ampulla, which contains an important 

 sensory apparatus. 



The sacculus, or lower division of the labyrinth, is an 

 irregular oval sac which projects downward and forward 

 from the utriculus, with which its interior is connected 

 through an aperture. There are four small outpocketings 

 from the base of the sacculus, and from the median side 

 there is given off a narrow tube, the ductus endolymphatictcs, 

 which extends dorsally where it penetrates the cranial cavity, 

 and ends in a large sac, the saccus endolymphaticus, which 

 is filled with a milky white fluid containing numerous white 

 crystals of carbonate of lime. The utriculus, the sac- 

 culus, and the larger outpocketings of the latter contain 



