52 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



/) The zygapophyses are anterior and posterior projections from the neural 

 arch, one in front of and one behind the place of origin of each transverse process. 

 They link successive vertebrae together. 



The first vertebra, or atlas, differs from the others in the absence of the trans- 

 verse processes and the presence of special articulating surfaces for holding the 

 skull. Between the atlas and the skull is a gap which permits the operation of 

 pithing. The transverse processes of the last vertebra articulate with the pelvic 

 girdle, and this vertebra is hence called the sacral vertebra. 



3. The skull and the visceral skeleton. The visceral skeleton includes the 

 upper and lower jaws, the hyoid apparatus, and the cartilages of the larynx. 

 It is so called because originally all of these structures were paired semicircular 

 cartilages, used to support the gills. Since gills are part of the walls of the 

 alimentary canal, the appropriateness of the term "visceral skeleton" becomes 

 apparent. Six such cartilaginous hoops are present in the frog tadpole, of which 

 the first becomes in the adult frog the basis of upper and lower jaws, while the 

 remaining ones undergo remarkable transformations into the hyoid and laryngeal 

 cartilages. The laryngeal cartilages will not be studied. 



The original cartilage skull, forming a case to inclose the brain, fused in front 

 with the olfactory capsules containing the organ of smell, behind with the otic 

 capsules containing the ears, and below with the cartilaginous bars which formed 

 the upper jaw while the similar bars of the lower jaw remained separate and 

 formed a joint with the skull. Part of the cartilage of these structures persists 

 in the adult skull, part is converted into cartilage bones, and both of these are 

 partly concealed by a superficial covering of membrane bones. 



a) Dorsal aspect of the skull and upper jaw (Holmes, chap, xiii, pp. 229-37). 

 Examine the dried skull and note that the 'membrane bones can be distinguished 

 easily from the cartilage bones by their lighter color, smoother surfaces, thin 

 and flat shapes, and more superficial position. The cartilage has of course dis- 

 appeared in a dried skull. The skull proper forms the central region, while the 

 lower jaw, which is fused to the skull, appears as an arch on each side and in 

 front of the skull. A large gap, the orbit, which holds the eye, is thus left between 

 skull and jaw. 



The bones of the dorsal side of the skull proper are : 



(1) Nasal bones, two triangular membrane bones just behind the external 

 nares. 



(2) Sphenethmoid bone, a single ring-shaped cartilage bone in the median 

 line behind the nasals. 



(3) Pronto parietal bones, two long flat membrane bones posterior to the 

 sphenethmoid. In other animals the anterior frontal portion is separate from 

 the posterior parietal portion of this bone. 



(4) Exoccipital bones, the two cartilage bones forming the posterior extremity 

 of the skull. They surround a large opening, the foramen magnum, through 



