2. THE SENSE CAPSULES. Each of the three special sense organs, of hearing, of sight, and of smell, is in the embryo provided with a cartilaginous or membranous protecting capsule; and two of these, the auditory and olfactory capsules, become afterwards more or less ossified, and intimately related to the cranium proper. (1) =Bones in relation to the Auditory capsules.= These bones lie on each side wedged into the vacuity between the lateral parts of the occipital and parietal segments; they are three in number, the =periotic=, the _tympanic_ and the _squamosal_. The =periotic= is the most important of them, as it replaces the cartilaginous auditory capsule of the embryo, and encloses the essential organ of hearing. It commences to ossify from three centres corresponding to the pro-otic, epi-otic and opisthotic of lower skulls, such as those of the Turtle and Crocodile. These ossifications however very early combine to form a single bone, the =periotic=, which nevertheless consists of two portions, the =petrous= and the =mastoid=, differing considerably from one another. [Illustration FIG. 74. DIAGRAM OF THE MAMMALIAN TYMPANIC CAVITY AND ASSOCIATED PARTS (modified from LLOYD MORGAN). 1. external auditory meatus. 2. tympanic membrane. 3. malleus. 4. incus. 5. lenticular. 6. stapes. 7. fenestra ovalis. 8. fenestra rotunda. 9. Eustachian tube. 10. cavity occupied by the cochlea. 11. cavity occupied by the membranous labyrinth.] The =petrous portion= lies dorsally and anteriorly, and is much the more important of the two, as it encloses the essential part of the auditory organ. It forms an irregular mass of hard dense bone, projecting into the cranial cavity, and does not appear on the external surface at all. The =mastoid portion= lies ventrally and posteriorly, is smaller, and formed of less dense bone than is the petrous portion, from which it differs also in the fact that it appears on the surface of the skull, just external to the exoccipital. The petrous portion bears a ridge, which together with a ridge on the supra-occipital, and the =tentorium= (fig. 72, 21), a transverse fold of the dura mater[142], separates the large cerebral fossa from the =cerebellar fossa=, which is much smaller than the cerebral fossa and lies behind and partly beneath it. The plane of the tentorium is called the =tentorial plane=, and the angles that it makes with the basicranial axis and with the occipital and ethmoidal planes vary much in different mammals.