220 HISTOLOGY 



Most of the vertebrate animals have a sense of hearing which is 

 made possible by the possession of auditory sense cells and all the 

 accessory tissues necessary to gather and intensify the sound waves 

 and transmit them to the perceptory cells. 



The perceptory cells in this case have been developed or evolved from 

 the static epithelium or from a common epithelium from which both of 

 these have originated. In the few other forms of animals that can hear, 

 the auditory tissues have originated from entirely different parts of the 

 body. 



The process of the evolution of auditory cells from the tactile or 

 static epithelium of the internal ear-sac is one whose progressive steps 

 can apparently be traced in the taxonomic series of vertebrates. Fishes 

 apparently are just coming into their power of hearing, and it is a question 

 if they can hear or not. Some probably can hear a few low sounds. 

 From the fishes up, the series of amphibia, reptiles, birds, and mammals 

 show successively higher stages in the development of a part of the ear-sac 

 into a cochlea or region of auditory perception, until, when we arrive at 

 the mammals, we find the beautifully arranged series of auditory cells 

 and accessory supporting cells which are grouped in a row which winds 

 in a spiral to save space. Like a snail shell, this hollow, spiral, tubular 

 part of the sacculus diminishes in size, and the resulting different lengths 

 of cells probably perceive lower or higher notes of sound. 



The auditory epithelium of the Guinea pig will serve as an example, 

 and we shall study the structure, although we cannot hope to entirely 

 understand the mechanism. Only the membranous portion, and espe- 

 cially its specific epithelium, will be treated of in this description, and 

 we shall begin by short embryological explanation-. 



The whole epithelium under consideration was originally a part of 

 the body epithelium on the sides of the head. At an early period (10 

 days in the rabbit) this epithelium thickened and was invaginated into 

 a sac with a narrow duct connecting it with the exterior. 



The sac continued to enlarge and the duct to close until it was cut 

 off entirely and obliterated. This left the sac as an internal cavity lined 

 with an epithelium. The sac enlarged and constricted in the middle 

 until it formed two sacs united by a duct. These two compartments 

 are called utriculus and sacculus, while the duct is known as the utriculo- 

 saccular duct in the adult. 



The utriculus now evaginated from its sides the three semicircular 

 canals, three curved tubes opening into the sac with both ends. Each 

 tube was enlarged into an ampulla which is described under the static 

 tissues. The epithelium of different parts of both sacculus and utriculus 

 were differentiated in several regions, while the whole complicated organ 

 was encased in a bony covering which formed around it. 



