1 86 VERTEBRATE SKELETON 



Notwithstanding the greater complexity, broad homologies can 

 be traced between the nasal capsules of mammals (fig. 198) and 

 reptiles. The capsules of the two sides are separated by a vertical 

 septum which extends back to the olfactory fenestra, lateral to the 

 septum, through which the first nerve enters the olfactory region. 

 At first each fenestra is simple in all mammals, and remains so in 

 Monotremes and some apes. Elsewhere it becomes divided into 

 numerous openings for separate branches of the nerve, then this 

 part is called the lamina cribrosa, the name (and also the word 

 ethmoid) referring to this sieve-hke condition. A roof of cartilage 

 extends laterally from the top of the septum over each nasal sac, and 

 bends downwards to form the lateral wall. 



In lower mammals the nasal cavities are in front of the cranial, 

 but in the higher the cranial overlaps the nasal, resulting in a change 

 in the position of the lamina cribrosa from nearly perpendicular to 

 approximately horizontal. 



When bones begin to develop in the skull, gaps (fontanelles) are 

 common in the fetal roof and sometimes persist in the adult. Most 

 common of these are the occipital fontanelle in the lambdoid suture 

 between the supraoccipital and the parietals; and the frontal fon- 

 tanelle in the coronary suture between parietals and frontals. Less 

 common is the asterion on either side between parietal, squamosal, 

 and occipital, and the pterion fontanelle between frontal, parietal 

 and alisphenoid. Vacuities are rare in the adult, exceptions being 

 most common in Ruminants between frontal, lacrimal and maxilla. 



The shape of the skull varies somewhat with age, this being most 

 marked in Primates, less in Carnivores. Cranial parts predomi- 

 nate over facial in higher groups, this not always caused by brain 

 development, for it also occurs in elephants, Ruminants and Edentates 

 as the result of cavities in the bones. Strong jaw muscles tend to 

 cause a crest in the sagittal line of the roof of the skull, this alTording 

 greater surface for muscular origin. It is noteworthy that, aside 

 from the ossicula auditus and the lower jaw, the bones of the adult 

 skull are immoveably united by futures, these being obliterated to 

 the greatest extent in Monotremes. 



The number of cartilage bones (fig. 199), distinct at first, is greater 

 than in modern reptiles; some, especially in the ethmoid and inter- 

 orbital regions, being without homologues in that group, but are 

 new acquisitions of mammals. 



