No. I.] THE VERTEBRATE EAR. IC| 



on removing the perilymphatic connective tissue. The anterior 

 vertical canal springs from the utriculus, and curving upward, 

 outward, and backward unites with the posterior vertical canal, 

 which, after arising from the posterior end of the sacculus and 

 passing upwards, forwards, curving as it does so, first outwards, 

 then inwards, meets its fellow above the median portion of the 

 utriculo-sacculus. They are somewhat flattened in section at 

 their ends, and open by wide mouths into the middle divisions 

 of their respective ampullar chambers. In the middle of their 

 course their cross-section is circular. Seen from the inside of 

 the utriculo-sacculus, the canals present the appearance of short 

 archways leading from the large ampullar chambers, below where 

 they freely communicate with the utriculo-sacculus up to the 

 narrowed ends of these same chambers, into which they again 

 open. The endolymphatic ducts, for Petromyzon possesses a 

 pair of them, both proceed from the utriculo-sacculus to the 

 cranial cavity. One of them opens into the lower posterior face 

 of the utriculus, while the other perforates the median triangular 

 plate between the utriculus and sacculus, with a tendency to 

 open more into one chamber than the other, and it is sometimes 

 the utriculus and sometimes the sacculus with which it most 

 freely communicates. The last-mentioned duct contains a large 

 sense organ (PI. I, Fig. 14, and PI. X, Fig. 2). 



In the Alligator the canals have begun to lose their impor- 

 tance. Passing from the Cyclostomes, where they begin their 

 differentiation, through the Elasmobranchs and true fishes, 

 where they reach their greatest perfection, we find them among 

 the Amphibia and all the air-breathers gradually losing their 

 prominence relative to the other parts of the ear, especially the 

 saccular region from which the lagena has begun its growth. 



I think it is not necessary to enter into a detailed description 

 of the semicircular canals here, although there is much of value 

 which might be recorded of these canals in the Sauropsid types. 



In Minms as a representative of the avian type and in the 

 mammalia as represented in Man, the canals are characterized 

 by an increased delicacy and regularity of outline, seldom show- 

 ing those modifications of form so apparent in the fish types. 

 In size the canals undergo considerable variation, and on the 

 whole, as said before, they are evidently of less importance to 

 the auditory economy than in the Ichthyopsida. In Mimus the 



