THE VERTEBRATE EAR. 



353 



EXPLANATION OF PLATE IX. 



Fig. I. A diagrammatic figure illustrating the sequence of sense organs or groups 

 of sensory cells in the organ of Corti of the mammalian ear. The measurements are 

 taken from the human ear, and the whole figure is magnified 8 diameters. I to 100 + 

 the number of the cell groups, or sense organs. The distribution of the cochlear 

 nerve and its breaking up into branches for the organs is taken from camera draw- 

 ings. Anastomoses are not shown, c.r., sacculo-cochlear canal, c.c, cochlear canal. 

 c.n., trunk of the cochlear nerve. 



Fig. 2. The Cyclostome type ear shown here is a very simple structure as com- 

 pared with the complicated organ found among the Gnathostomata, but compared 

 with its ancestral condition, i.e. the auditory vesicle, it is seen to have gone through 

 many changes, and stands to-day midway between its earliest condition and the high- 

 est differentiation known. This diagram is constructed on the basis of the anatomy 

 of the only two known forms of this group, Myxine and Petroniyzon. The canals 

 and their organ retain a very primitive condition of structure. 

 caa., anterior canal. 



cap., posterior canal. 



cr.a., crista acustica anterior. 



cr.p., crista acustica posterior. 



d., endolymphatic duct. 



U.S., the utriculo-sacculus. 



u. I and 2, branchlets of the utricular branch of the auditory nerve. 

 s. I and 2, the saccular branches of the same. 

 C.S., sense organ of the canal commissure. 



n.ac.f., accessory auditory nerve. 



ode., terminal portion of the endolymphatic ducts, which in neither of these 



Cyclostomes open on the surface of the body in adult life. Only one, 



the utricular duct, is represented as bearing canal organs, though it 



is nearly certain that both ducts are sense organ canals in Petromyzon. 



tnac.u., macula acustica utriculi. 



macs., macula acustica sacculi. 



I and 2, the portions (i) of the sense organs that remain in the parent cavity 



and the portion (2) which migrate into the recessus utriculi and 



lagena respectively. 



Fig. 3. The Gnathostome type ear shown here illustrates the method and the 



extent of the modifications which the Cyclostome ear has suffered in its descent to 



this group. The anterior and posterior canal organs have each budded off an organ 



which, in the case of the anterior canal, produces a well-formed canal, the external, 



present in all the members of this group, while the posterior bud never produces a 



well-formed canal. The utricular recess, which contains a number of sense organs 



derived from the division of the macula utriculi, is represented as forming a tubular 



prolongation of the utricular cavity, in order to more clearly show the parallelism of 



this group of organs with the lagenar group. In no known form does the production 



of the canal proceed to the extent shown in the diagram. The sense organs of the 



