THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 115 



anterior end projects as a thin blunt process, the rostrum, into the space between 

 the two orbits. Find it in side view of the skull. Above the rostrum on each 

 side is an alisphenoid bone, extending upward to the roof of the skull and forming 

 part of the walls of the foramen ovale and the supratemporal fossa. In the 

 interior of the skull is a cavity extending from the foramen magnum to the ali- 

 sphenoid bones; this cavity contains the brain in life, and the ventral surface of 

 the brain therefore rests upon the occipital and sphenoid bones, derived from the 

 parachordals and prechordals. 



7. The anterior sphenoid and ethmoid regions. These regions remain in the 

 cartilage stage in the alligator, forming a cartilaginous partition between the 

 two nasal passages and between the orbits. The cartilage is of course wanting 

 in the dried skull so that the nasal passages and orbits appear to be connected. 



Draw a ventral view of the skull, showing the sutures accurately. 



8. The lower jaw. The lower jaw or mandible is composed of two halves or 

 rami united in front by a symphysis. Each ramus consists of six separate bones. 

 Near the posterior end of the jaw is a large vacuity, the external mandibular 

 foramen, and in front of this on the inner side a smaller internal mandibular 

 foramen. The bones of each ramus are: the dentary, bearing teeth and forming 

 the outer surface of the anterior two-thirds of the ramus; the splenial, of about 

 the same shape and size as the dentary and in the same position on the inner 

 surface; the angular, below the external mandibular foramen, and separated 

 from the splenial by the internal mandibular foramen; the supra-angular or 

 surangular, above the external mandibular foramen; the coronoid, a small bone 

 on the inner surface, between the anterior ends of the angular and the supra- 

 angular; and the articular, above the posterior end of the angular and bearing 

 a concavity for articulation with the quadrate. A cavity exists in the interior 

 of the ramus between the dentary and the splenial. This cavity is occupied in 

 life by Meckel's cartilage, which it may be recalled is the original lower half of 

 the first gill arch. The posterior end of this cartilage has ossified into the articular, 

 which is therefore the only cartilage bone in the lower jaw. The articulation 

 between upper and lower jaw is by way of the articular and the quadrate, a 

 condition found in the majority of land vertebrates. Draw the lower jaw from 

 both inner and outer views. 



9. The hyoid apparatus. As has already been explained, this is derived 

 from the hyoid arch and remaining gill arches. The hyoid apparatus is generally 

 missing on dried skeletons, and for its study preserved specimens are necessary. 

 It consists of a broad cartilaginous plate, the body of the hyoid, and a pair of 

 processes or horns (cornua) extending posteriorly and dorsally from the body, 

 one on either side. The cornua are partially ossified. The body of the hyoid 

 apparatus is derived trom the bases of the second and other arches while the 

 horns are remnants of the third arch. In lieu of alligator material, the hyoid 

 apparatus may also be studied on turtle skeletons. In the turtles there is a 



