292 



COMPARATIVE ANATOMY 



se 



and these lie in planes roughly at right angles to one another. 

 They are distinguished as the anterior vertical, the posterior vertical, 

 and the horizontal (external) canal (Fig. 213). The first and last- 

 named arise from the portion of the utriculus known as the recessus 

 utriculi, and at its origin each has a vesicle-like swelling or 

 ampulla, enclosing sensory cells. The posterior canal also arises 

 with an ampulla from a prolongation of the utriculus. The other 

 end of the horizontal canal opens by a funnel-shaped enlargement 



into the utriculus, while that of 

 the anterior and of the posterior 

 canal unite to form a common 

 tube, the so-called canal commis- 

 sure (sinus superior), which also 

 opens into the utriculus. 



Concretions composed mainly 

 of carbonate of lime are present 

 in the regions of the various 

 nerve end-plates of the auditory 

 organ in all Vertebrates. These 

 otoliths, which arise in the epi- 

 thelium lining the organ, and 

 are then set free into its cavi- 

 ties, present the greatest variety 

 both in form and size. The 



ass 



ecu 



aa 



rec 



FIG. 213. SEMIDIAGRAMMATIC FIGURE 



largest and most massive ones 

 are seen in Teleosts. They 

 either consist of a single mass. 



OF THE LEFT MEMBRANOUS LABY- r separate otoliths are present 

 RINTH OF A VERTEBRATE. 

 from the outer side. 



As seen i n different regions of the laby- 

 rinth. 



The sensory epithelium, to 

 which the branches of the audi- 

 tory nerve are distributed, is 



utriculo-saccular canal ; de, se, ductus situated in the following parts of 



and saccus endolymphaticus, the the membranous labyrinth : (1) the 



former arising from the sacculus at t h re e ampulla of the canals, in 



each of which the auditory cells 

 are situated on a ridge (crista 

 acustica) projecting into the lumen 

 (Fig. 214); (2) the utriculus 



and the recessus utriculi ; (3) the sacculus and lagena, or rudiment 

 of the cochlea ; (4) the rudimentary macula acustica neglecta, which 

 in Fishes, Birds, and Reptiles is situated on the floor of the 

 utriculus close to the sacculo-utricular canal, in Amphibians on the 

 inner side of the sacculus, and in Mammals undergoes a gradual 

 reduction and may even become obliterated. 1 The several portions 



1 In addition to these, there is a transitory macula ductus reunientis situated 

 in the region of the sacculo*cochlear duct. 



ass, apex of the sinus utriculi superior ; 

 ca, ce, cp, anterior, external, and 

 posterior semicircular canals ; aa, ae, 

 ap, the corresponding ampullse ; cus, 



t ; I, recessus sacculi (lagena) ; rec, 

 recessus utriculi ; 8, sacculus ; sp, 

 sinus utriculi posterior ; ss, sinus 

 utriculi superior ; u, utriculus. 



