50 AYERS. [Vol. VI. 



here to recall the fact of their very general occurrence. Of the 

 causes which have led to their enormous development in fishes, 

 amphibia, and reptiles, with their reduction among birds and 

 mammals, we know nothing definite. The physico-physiological 

 conditions and consequences of this development I have not 

 investigated. 



CONTINUITY OF CEREBRAL AND AUDITORY SPACES. 



According to Schwalbe, the subdural space in Man com- 

 municates with the perilymphatic spaces through the porus 

 acusticus, and Key and Retzius found that not only the sub- 

 dural, but also the subarachnoidal space was accessible through 

 the perilymphatic channels. 



Weber-Liel has shown experimentally that the perilymphatic 

 spaces of the ear are connected with the subarachnoidal spaces 

 about the brain. 



The perilymph, according to Hasse, flows principally through 

 a membranous canal, ductus perilymphaticus, through the fora- 

 men jugulare into the lymph channels at the base of the head, — 

 some part of it passes in through the porus acusticus internus 

 into the subdural spaces about the brain. The endolymph flows 

 out through the arachnoidal sheath of the auditory nerve into 

 the subarachnoidal spaces, and renews itself perhaps by trans- 

 fusion through the ductus and saccus endolymphaticus. The 

 continuity of these spaces is much more distinct in the fishes 

 than in the higher forms. 



COCHLEAR ORGANS OF THE MAMMALIA. 



PI. Ill, Figs, r and 2 ; PI. V, Fig. 9 ; PI. VI, i ; PL X, Figs. 

 3 and 4 ; and PI. XII. 

 The Sauropsid organ. 

 Corti's organ in the Mammalia. 



a. Relation of component structures and of the cellular elements. 



The cochlear frame. 



b. Canal organ arrangement of the cell groups and the composi- 



tion of a group. 



c. The inner and outer hair cells. 



d. The supporting cells. Pillars of Corti. 



e. The hair band or membrana tectoria and the mb. reticularis. 



