758 MAMMALIA. 



in the Reptile ; but in the Mammifer these are at an early period con- 

 solidated into one mass, situated at the back of the cranium. Its basilar 

 portion (5) articulates with the atlas by two condyles ; while the lateral 

 wings (10) and the superior arch (8) surround the foramen magnum, 

 and protect the cerebellic regions of the encephalon. 



(2168.) The sphenoid (6), although composed of fewer separate pieces 

 than in the Reptilia, and even regarded by the human anatomist as a 

 single bone, is still distinctly divisible, especially in young animals, into 

 two very important portions one anterior, and the other posterior 

 each, as we shall soon see, forming the body of a distinct cranial vertebra. 

 The posterior half (6) consists of the body, including the posterior 

 clinoid processes, and of the greater alee and pterygoid processes 

 (fig. 381, 25). The anterior half is formed by the anterior clinoid pro- 

 cesses and alae minores (fig. 381, 11). These two halves may therefore 

 be called, respectively, the anterior and posterior sphenoids. 



(2169.) Lastly, we have the temporal lone, exhibiting but one piece, 

 although made up of all the parts which in the Reptile were so ob- 

 viously distinct elements. The petrous portion, wedged into the base 

 of the cranium, still encloses the internal car. The tympanic element 

 (fig. 380, a) supports the membrana tympani. The mastoid process 

 (fig. 381, 12) is the homologue of the mastoid bone of the Crocodile ; 

 and, lastly, the squamous element, with which the lower jaw is articu- 

 lated (fig. 380, 23), in the Reptilia was visibly a distinct bone. Even 

 to these may be added the zygomatic process, which Professor Owen 

 regards as an independent elemental part. 



(2170.) Reviewing, therefore, all that has been said relative to the 

 composition of the skull in the differen^ classes of Yertebrata, the fol- 

 lowing deductions may be arrived at */ 



1. That as we advance from lower to higher forms, the proportionate 

 size of the cranium relative to that of the face becomes greater. 



2. That the number of bones met with upon the inferior and lateral 

 aspects of the head gradually diminishes : for in Mammalia the ptery- 

 goid and tympanic bones, which even in Birds are separate pieces, 

 become very generally confounded with the sphenoid and the temporal : 

 and also the petrous and squamous portions of the temporal become 

 blended together. 



3. The number of bones normally entering into the composition of the 

 cranium of adult Mammalia varies considerably. When most numerous, 

 there are twenty-eight eleven in the cranium, and seventeen in the 

 face. In this case the cranial bones are the following : one occipital, 

 one sphenoid, the two squamous portions of the temporal, the two 

 tympano-petrous portions of the temporal, the two parietal, the two 

 frontal, and the ethmoid. 



(2171.) The bones of the face are : two superior maxillary, two inter- 

 * Meckel, Traite G6nerale d'Anatomie Comparee, torn. iii. 2 de part., p. 195. 



