SKULL 429 



cartilage remains. The surface of the nasal cavities is much 

 increased by three pairs of thin, rolled cartilage bones known as 

 turbinals. The membrane bones of the capsule are a pair of nasals, 

 which form the roof of the nasal cavities, and a median vomer, 

 which is formed from paired elements and has a forked vertical 

 part partially enclosing the lower edge of the cartilaginous nasal 

 septum, and lateral wings which separate the nasal cavities into 

 upper olfactory chambers which are blind, and a lower nasal 

 passage which opens posteriorly into the pharynx. 



JAWS 



The ventral part of the skull, consisting of the jaws and visceral 

 arches, is much better seen in the dogfish, where it is reasonably 

 complete. In mammals little is left of it but the jaws. 



The cartilaginous upper jaw in the embryo is known as the 

 palatopterygoquadrate bar, because in some lower vertebrates it 

 becomes ossified to form three bones called palatine, pterygoid, 

 and quadrate. In mammals, however, most of the cartilage 

 breaks down and disappears, leaving bone only in the middle 

 and at the posterior end. The middle portion forms the ali- 

 sphenoid, which we have already seen forming part of the side 

 wall of the brain-box, and the posterior quadrate becomes the 

 incus, one of the ear ossicles. The functional upper jaw is thus 

 made entirely of membrane bones. The bone usually called 

 pterygoid, but better called ectopterygoid to show that it is 

 not the cartilage bone of that name, projects downwards as a 

 triangular plate from the junction of the basisphenoid with the 

 alisphenoid. In front of this the dermal palatine stretches forwards 

 below and at the side of the presphenoid. Its anterior part is 

 bent inwards and meets its fellow in the middle line, so that 

 the narial passage is extended backwards and the internal nares 

 open far back in the buccal cavity. The secondary roof to the 

 mouth formed by this plate of bone is called the false palate. In 

 front of and outside this series of upper jaw bones is a second set. 

 Running forward from the zygomatic process of the squamosal, 

 and completing the zygomatic arch, is the malar or jugal. It joins 

 in front a short zygomatic process from a large bone called the 

 maxilla. This makes up most of the face, and joins the nasal, 

 lacrimal, and frontal above. Below, it continues the false palate 

 made by the palatine, and meets its fellow in the middle line. 



