MAMMALIA 261 



capsule is also chondrified and the cartilage united with that 

 of the brain case. In front of the brain case chondrification 

 proceeds with the development of the nose. The nose is com- 

 pletely invested in cartilage above, but below is produced into 

 processes bounding the wide foramen of the posterior nares. 

 The nasal capsule provides an opening for the anterior nares, 

 and behind, where it enters into the formation of the orbit, it 

 is perforated by the olfactory nerves. It is divided into two 

 chambers by a septum, and the chambers are modified by the 

 ingrowth of the turbinals, laterally and posteriorly. It is 

 thus evident that behind the nose the cartilaginous skull is 

 widely open, presenting a large fontanelle. It is replaced by 

 the bony skull, cartilage bones being formed by replacement 

 and membrane or covering bones, developed independently. 



Visceral arches are produced at the same time. The first 

 or mandibular arch is formed as a Meckel's cartilage, a malleus 

 and incus. The hyoid arch is resolved into a stapes and the 

 anterior cornu of the hyoid. The branchial arches succeed, 

 but disappear with the exception of the first, which persists as 

 the posterior cornu of the hyoid. 



The skull of the dog is now commonly used for the study of 

 the adult condition of the skull, but any other convenient skull 

 may be utilised. 



The skull is at once resolved into the fused elements, forming 

 the brain case and face, the movable lower jaw, and the hyoid. 

 In section and in superficial view the following bones and their 

 relationship to one another will be easily made out. The 

 frontals form the upper protecting bones of the anterior region 

 of the brain case. They expand into small blunt supraorbital 

 processes, and are bent downwards and inwards to form a 

 large part of the orbit. In section, the frontals are hollow; 

 the cavity called the frontal sinus communicates with the 

 nasal cavity, and is consequently filled with air. The parietals 

 roof a large area of the brain. They take part with the frontals 

 in providing a crest for the upper margin of the temporal 

 muscle, a muscle which runs to the lower jaw. Posteriorly 

 the interparietal is interposed between the parietals. Both 

 the frontals and parietals are membrane bones. 



The occipitals and the basal bones of the skull are cartilage 



