THE SKTLL. 



217 



the external auditory uieatus, and is frequently dilated to a projecting 

 capsule (tympanic bulla). Postfrontals are absent. The perforated 

 >:i-//>i-i/<>i-iit plate (lamina cribrosa) of the ethmoid forms the anterior 

 boundary of the cranial cavity. In the Apes and Man only, do the 

 lateral parts of the ethmoid (the part known as l<nnni. papyracea) 

 take part in the formation of the inner wall of the orbit. In all 

 other cases the ethmoid is placed in front of the orbit, and its side- 

 are covered by the maxillaries ; in such cases it has a considerable 

 longitudinal extension. Two parts may be distinguished in the 

 ethmoid (1) A median plate the lamina perpendieula/iris which 

 is continued in front into the cartilaginous internasal septum, and 

 is underlaid by the vomer; (2) the lateral masses, with the lamina 

 crittrosa and the labyrinth (ethmoidal cells and the two upper tur- 

 binals) ; the first corresponds to the unpaired ethmoid, the second to the 

 prsef rentals of the lower Vertebrates. Finally, in the anterior part 

 of the nasal cavities there are, as independent ossifications, the inferior 

 turbinals ( maxillo-turbinals), which are attached to the inner surfaces 

 of the maxillary bones. On the outer surface of the ethmoid region 

 are placed, as membrane bones, the nasals above and the lachrymals 

 to the sides. The lachrymal (absent as an independent bone in the 

 Pinnipedia and most Cetacea) is placed in the anterior wall of the 

 orbit ; but usually also appears as a facial bone on the outer surface. 



The firm fusion of the maxillo-palatine apparatus with the skull 

 and the relation of the mandibular suspensorium to the tympanic 

 cavity, are characteristic of the Mammalia. The lower jaw articu- 

 lates directly with the temporal bone without the interposition of n. 

 quadrate, the morphological equivalent of which is shifted, in the 

 course of development, into the tympanic cavity and transformed 

 into the incus, while the upper part of Meckel's cartilage (articulare 

 of the lower jaw) becomes the malleus (Reichert). The stapes, on 

 the contrary, is said to be developed from the upper piece of the 

 hyoid arch (hyomcvndibular). The maxillary, pterygoid, and palatine 

 bones have similar relations to those of the CJidonia and Crocodilia, 

 but a quadratojugal is always wanting, since the jugal is applied to 

 the squamosal. A palatal roof (hard palate) sepai-ating the buccal 

 and nasal cavities is always present ; the posterior nares open at its 

 hind end. 



The cranial capsule is so completely filled by the brain in the 

 Mammalia that its internal surface presents a relatively accurate 

 impression of the surface of the brain. Owing to the considerable 

 size of the brain the cranial capsule is far more spacious than in any 



