126 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



12. The hyoid apparatus. This is, as already explained, the remnant of the 

 hyoid and other gill arches. It is generally absent on prepared skeletons and 

 isolated specimens will be provided for its study. 



The hyoid of the cat consists of a bony bar placed in the root of the tongue 

 just in front of the larynx; this bar is called the body of the hyoid. From it 

 extend two pairs of processes or horns, an anterior pair and a posterior pair. 

 The anterior horns consist of four separate pieces, of which the terminal piece 

 is attached to the tympanic bulla just ventral to the stylomastoid foramen 

 of the skull. The groove which it occupies can generally be seen on the side of 

 the bulla. The posterior horns consist each of a single piece which is united to 

 the larynx. The body and anterior horns belong to the hyoid arch ; the posterior 

 horns to the third gill arch. The remaining gill arches are represented in the 

 larynx which will be studied later. 



The hyoid apparatus of the rabbit consists of a stout bone, the body of the 

 hyoid, situated at the base of the tongue in front of the larynx. It bears two 

 pairs of processes or horns. The anterior horn is a short piece connected by a 

 muscle with the jugular process of the skull. The posterior horn is a longer 

 piece connected by ligament with the larynx and by muscle with the jugular 

 process. The body and anterior horn are remnants of the hyoid arch, the posterior 

 horn of the third gill arch. 



H. GENERAL SUMMARY OF THE SKULL AND VISCERAL SKELETON 



1. The skull begins as a cartilaginous case, the chondrocranium, whose method of 

 formation has already been described. This case includes the olfactory and otic 

 capsules. 



2. The gill arches become closely associated with the chondrocranium. There are 

 usually seven of them, the first forming the upper and lower jaws, the second the hyoid arch. 

 The arches are reduced in number and altered in function in land vertebrates. 



3. Bones derived from the chondrocranium and gill arches are the cartilage bones of 

 the skull. 



4. In addition to these there are membrane bones added. They come from the dermis 

 of the skin and were originally dermal scales. 



5. The cartilage bones of the skull and sense capsules are the occipitals, sphenoids, 

 ethmoids, otics, and turbinals. There are four occipitals (supraoccipital, two exoccipitals, 

 basioccipital) ; a posterior group of three sphenoids (basisphenoid, two alisphenoids) ; an 

 anterior group of three sphenoids (presphenoid, two orbitosphenoids) ; and a group of three 

 ethmoids (mesethmoid, two ectethmoids). The otic bones ossified in the ear capsule are 

 generally three in number but may be as many as five; in mammals they are fused to form 

 a periotic or petromastoid bone. The nasal capsules may furnish additional cartilage bones. 

 The occipitals and sphenoids form the posterior end and floor of the brain cavity, while the 

 ethmoids inclose the nasal cavities. 



6. The first gill arch generally gives rise to two cartilage bones, a quadrate derived from 

 the posterior end of the upper jaw cartilages or pterygoquadrate cartilages; and an articular 

 derived from the posterior end of the lower jaw cartilages or Meckel's cartilages. In some 

 forms the pterygoquadrate cartilages may give rise to palatines and pterygoids in addition. 



