296 AN INTRODUCTION TO ZOOLOGY 



and coiled series of bony lamellae consituting the so-called ethno- 

 turbinal bones, since they are fused behind with the ethmoid bone. 

 They are covered by the olfactory membrane, which is well supplied 

 with sense cells and nerves, and so serve to provide a much greater 

 area for the sensory epithelium than would be possible without the 

 folding. The most dorsal of these lamellae lie close to the nasal bones 

 with which they become fused in some mammals, and then they are 

 termed the naso-turbinals. A similar series of even more slender 

 lamellae are to be found in the anterior part of the nasal cavity, and 

 as they are fused with the maxillae they are distinguished as the 

 maxillo-turbinals. 



The optic capsule, well developed in bony fishes, is very much 

 reduced in the mammal and comes to consist of a single membrane 

 bone, the lachrymal, on each side. This is a small bone lying on 

 the anterior border of the orbit at the ventral limit of the junction 

 between the frontal bone and the maxilla. It is perforated by the 

 lachrymal foramen, through which passes the duct of the lachrymal 

 or tear gland. In some mammals it is but loosely attached to the 

 surrounding bones, but in the dog it is more firmly inserted. ' 



The auditory capsule consists of a group of bones more or 

 less firmly joined together and lying laterally to the basi-occipital 

 and immediately in front of the par-occipital process. The most 

 obvious part externally is the tympanic, a cartilage bone which 

 is swollen out ventrally to form the well-marked tympanic bulla. 

 At the hinder median border of the bulla lies an obliquely directed 

 aperture, the foramen lacerum posterius, through which the ninth, 

 tenth and eleventh cranial nerves leave the skull in company with 

 the internal jugular vein. Behind its postero-dorsal corner is a 

 somewhat round hole, the stylomastoid foramen, serving for the 

 exit of the main part of the seventh cranial nerve. Antero-laterally 

 to the bulla lies an irregular opening, the foramen lacerum medium, 

 which on closer examination is seen to be double. The more median 

 of the two perforations is the foramen caroticum, through which 

 the internal carotid artery reaches the brain and slightly to the 

 outside of this is the orifice of the Eustachian canal for the 

 Eustachian tube. On its anterior, upper surface the tympanic 

 bone is produced into a short neck with a wide opening, the external 

 auditory meatus. Across the bottom of the neck in life is stretched 

 the tympanic membrane supported by an incomplete bony ring. 



The internal portion of the auditory capsule is formed by an 

 irregular cartilage bone, the peri-otic, which is regarded as being 

 composed of three separate elements, the pro-otic, the epi-otic and 

 the opisth-otic, and lodges the internal ear. In the adult the single 

 bone formed by this fusion can be divided into two more or less 



