AUDITOEY ORGANS OF VERTEBRATA. 535 



one another more or less perpendicularly; the third, and outer one, lies 

 in a more horizontal plane, and is provided with an ampulla on its 

 posterior limb. The two vertical canals have a common piece (c) 

 which opens into the vestibule, and ampullae at the two other ends. 



Even in Fishes the vestibule of the labyrinth is divided into 

 several portions. An upper one is directly connected with the 

 semicircular canals (utriculus, alveus communis), and with the 

 subjacent sacculus. The sacculus aud utriculus contain otoliths, 

 which are constant in the same, but different for different, divisions ; 

 they are often very large. The branches of the auditory nerve pass 

 into the end-organs which are to be found in the walls of both 

 cavities, as well as in the ampullae of the semicircular canals ; in 

 the ampullae they are placed on a transverse ridge (crista acustica) ; 

 in the saccules they form the maculae acusticae. 



Of the numerous modifications which may be observed, the 

 connections between the membranous vestibule and the air-bladder 

 are worthy of remark ; the arrangement is effected in various ways ; 

 it is simplest in some of the Percoidea, and Sparoidea, where the 

 vestibule is continued into spaces in the skull, which are merely 

 covered by membrane ; to these spaces processes of the air-bladder 

 are attached. The relations are more complicated in many families of 

 the Physostomi. In the Cyprinoids the sacculus (a) extends back- 

 wards, and is connected with that of the other side by a sinus impar. 

 This gives off a membranous saccule (atrium sinus imparis) on either 

 side, which passes to an opening on the posterior portion of the 

 skull, which is partly closed by a small bone. This is connected by 

 masses of ligament with a series of bony pieces ( i Id) of various 

 forms, the last and largest of which is attached to the anterior end 

 of the air-bladder (m). These ossicles are modifications of ribs, and 

 form a continuous chain between the vestibule and the air-bladder. 

 In the Siluroidea and Clupeidea connections with the air-bladder 

 are effected in a different manner. 



§ 401. 



In and above the Amphibia, the labyrinth is greatly diminished 

 in size from what it is in Fishes. It is still of a relatively large 

 size in the Amphibia, and is smallest in the Mammalia. The dif- 

 ferences which are seen in it are partly due to the way in which the 

 two cavities of the vestibule, the utriculus and sacculus, are connected 

 together, and to the course taken by the semicircular canals which 

 spring from the former. The posterior canal may sometimes be set 

 at an angle to the external one (Birds). 



There is a great difference between the portion of the labyrinth, 

 just described, and which is very similar in all forms, and that part 

 which is only developed as an independent structure in the higher 

 divisions; this, which is known in Mammals as the cochlea, on 

 account of its form, presents a continuous series of differentiations 



