88 bulletin: museum of comparative zoology. 



has only a ligamentous connection with the surrounding bones of the 

 skull, and hence readily becomes detached from the latter in maceration. 

 The body of the tympanic corresponds to the bulla tympanica of various 

 other Mammalia, that of the periotic, which somewhat resembles the 

 figure 6 in form, to the petrous bone. Processes are developed by each 

 by means of which the two elements are conjoined at either end, leaving 

 a narrow longitudinal slit between them, known as the " tympano- 

 periotic fissure." Although anchylosed in the adult, thus justifying the 

 term of tympano-periotic, the two bones are easily separable in young 

 individuals. During early stages, also, according to Boenninghaus,^ 

 these bones form almost an integral part of the periotic region of the 

 cranium, and are oriented with their long axis parallel with the median 

 line of the body, the spout-like aperture for the Eustachian canal being 

 placed foi'emost. The elements soon become protruded downward, 

 however, and a recess is formed for them on the side of the base of the 

 cranium ; owing to the more rapid increase in width of the hinder portion 

 of the skull, the main axis of the tympano-periotic becomes shifted so as 

 to be directed almost diagonally with reference to tlie longitudinal axis 

 of the body. Notwithstanding this obliquity of position, it is cus- 

 tomary to speak of the two extremities as anterior and posterior respec- 

 tively, and of the lateral walls or " lips " of the bulla as external and 

 internal. Tlie latter bone is hollow, broad, rounded, and distinctly 

 bilobed behind, pointed in front, and open above except for a short dis- 

 tance posteriorly where the process for attachment with the periotic 

 forms an archway spanning over both lips. Immediately in front 

 of this process is a somewhat crescentic opening for the external 

 auditory meatus, closed in the living animal by the membrana 

 tympani. 



The periotic is an irregular bone, somewhat shorter than tlie tym- 

 panic ; its central rounded portion or promontory, which encloses the 

 cochlea, is very dense, and pierced by several openings. On the cere- 

 bral side is seen the large meatus auditorius internus ; on the surface 

 opposed to the tympanic cavity the fenestra oralis (or vestihuli), which 

 receives the stapes, and directly above it the aperture for the Fallopian 

 canal ; nearly in line with them below, and looking posteriorly, is the 

 somewhat larger fenestra rotunda, otherwise known as the fenestra 



Journ. Anat. Physiol. (1894), 30, p. 366-413. Denker, A., Zur Anatomie des 

 Geliororgans der Cetacea. Anat. Ilefte (1902), 19, p. 423-448. Boenninghaus, G., 

 Das Olir des Zahnwales. Zool. Jahrb. Anat. (1904), 19, p. 189-360. 

 1 Loc. cit., p. 225. 



