58 ANATOMY OF THE RABBIT 



into a definite pattern of surface plates, having in general the same 

 disposition as derm elements in the skull of all higher vertebrates, 

 and distributed in such a way that they form an almost complete 

 enclosure for those parts of the chondrocranium which they invest. 

 The pattern of the dorsal elements is best indicated in fossil am- 

 phibia and early reptilia, in which the plan is almost diagrammatic. 

 The elements of the head skeleton may be classified as follows: 



1. The CEREBRAL CRANIUM (cranium cerebrale or neuro- 

 cranium), including: 



(a) The primary cartilage skull (chondrocranium), enclosing 

 the brain, and containing in its wall the olfactory and 

 auditory capsules (embryonic) ; 



(b) The secondary bone skull (osteocranium), replacing (a) 

 and comprising the occipital, sphenoid, ethmoid, in- 

 ferior turbinal, and periotic bones; 



(c) The associated derm elements, comprising the inter- 

 parietal, parietal, frontal, nasal, vomerine, lacrimal, 

 tympanic,^ and squamosal bones. 



2. The VISCERAL CRANIUM (cranium viscerale or splanch- 

 nocranium), including: 



(a) The primary mandibular and hyoid- visceral arches 

 (embryonic); 



(b) The secondary elements, replacing (a) — the malleus, 

 incus, and stapes of the auditory chain ; the hyoid bone 

 and its connections with the skull; 



(c) The associated derm elements of the face and palate, 

 comprising the premaxillary, maxillary, zygomatic, 

 mandibular, palatine, and vestigial pterygoid bones. 



Dentition 



Accessory structures intimately associated with the visceral 

 skeleton, though related functionally to the digestive system, are 



^The identification of the tympanic as a derm element has been questioned. 



^The thyreoid cartilage of the larynx and its connection with the hyoid 

 (greater cornu) and possibly the other laryngeal cartilages are modified bran- 

 chial arches, but the structure as a whole is not included with the head skeleton. 



