104 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



bones, derived, as we have already seen, from the dermal scales of the ganoid fishes. In a 

 similar way certain cartilage bones arise in the mandibular and hyoid arches and become 

 sheathed in membrane bones. The complete vertebrate skull, therefore, consists of cartilage 

 bones derived from the chondrocranium and its sense capsules, and the first and second gill 

 arches, and of membrane bones covering the cartilage bones everywhere except on the ventral 

 surface and posterior end of the skull. We may now state in detail the bones derived in these 

 ways to form the skull. 



1. Cartilage bones derived from the pre- and parachordals. There are four groups of 

 such cartilage bones: one occipital, two sphenoid, and one ethmoid group (see Fig. 35). 



a) Occipital group: This consists of four occipital 

 bones situated at the posterior end of the skull, 

 forming a ring around the foramen magnum. They 

 are one supraoccipital, two exoccipitals, and one 

 basioccipital. They arise from the parachordal car- 

 tilages, together with their dorsal extension the 

 synotic tectum and the vertebral elements which 

 are fused to the posterior end of the parachordals 

 (Fig. 35). 



b) Posterior sphenoid group: This comprises 

 three sphenoid bones, on the ventral side of the 

 skull, in front of the basioccipital. They are the 

 median unpaired basisphenoid and the paired ali- 

 sphenoids, one on each side extending up into the 

 back wall of the orbit. These bones come from the 

 posterior part of the prechordals. 



c) Anterior sphenoid group: This includes three 

 bones on the ventral side anterior to the preceding 

 and also derived from the prechordal cartilages. 

 They are a median unpaired presphenoid and paired 

 orbitosphenoids, one on each side forming part of 

 the walls of the orbit. 



d) Ethmoid group: This group consists of three 

 bones at the anterior end of the ventral side of the 

 skull, just behind and fused with the olfactory cap- 

 sules. The bones are a median mesethmoid and 

 paired ectethmoids, one on each side of the preceding. 

 The mesethmoid forms a median partition or sep- 

 tum between the two olfactory capsules, while the 



ectethmoids contribute to the posterior walls of these capsules. The ethmoids arise in the 

 ethmoid plate, which represents the fused anterior ends of the prechordals. 



2. Cartilage bones derived from the sense capsules. 



a) From the otic or auditory capsules: A number of otic bones arise in the walls of the 

 otic capsules. Since the latter are fused with the parachordal region of the skull, the otic 

 bones are naturally closely associated with the occipital group of bones and often fused with 

 them. There may be as many as five otic bones (in teleosts), the prootic, epiotic, opisthotic, 

 pterotic, and sphenotic, but they are commonly fused together or fused with nearby bones. 

 The three first named are the ones most constant in the higher vertebrates. When fused 

 into one bone they are designated as the periotic or petromastoid bone. 



b} From the optic capsules: As already explained, the optic capsules do not fuse with the 

 skull, owing to the necessity for retaining free movement in the eyes. The optic capsule 



exoccipital 

 supraoccipital 



FIG. 35. Diagram to show the ar- 

 rangement of the cartilage bones of the 

 skull, seen from above. The separate 

 otic bones, which may be as many as five 

 in number, are not represented. The 

 supraoccipital forms an arch above the 

 basioccipital. (Modified from Kingsley's 

 Comparative Anatomy of Vertebrates.) 



