232 COMPARATIVE ANATOMY 



cavity and join the chain of earbones as the malleus. By a similar piracy 

 in the upper jaw, the squamosal replaced the quadrate which also was freed 

 to pass into the tympanic cavity and become the incus. (Fig. 190) 



The evidence, therefore, compels us to believe that the skeletal ele- 

 ments which primarily functioned as supports of the visceral arches in 

 connexion with respiration have been converted through evolutionary 

 change into organs of mastication, sound-conduction, speech, and support 

 of tongue muscles. That the cartilage of the external pinna of the ear 

 is a derivative of the hyoid arch has been asserted on the basis of doubtful 

 evidence. 



The Visceral Skeleton in Man. The visceral skeleton is that part of 

 the axial skeleton which is related to the mouth and pharynx. In man it 

 includes upper and lower jaws, the hyoid bone, the cartilages of the voice 

 box, and the ear bones, malleus, incus, and stapes. The maxilla or upper 

 jaw is really a pair of bones fused together along the middle line. It is 

 nearly cubical in shape, has a body and four processes and encloses the 

 maxillary sinus. The upper surface of the maxilla forms the lower surface 

 of the orbit of the eye. From its inner surface the inferior concha projects 

 into the nasal passage. 



Of its four processes, the frontal articulates with the frontal bone, the 

 zygomatic with the zygomatic bone, the palatine with the palatine bone, 

 while the alveolar process bears the sockets for the teeth of the upper jaw. 

 Near the incisor teeth each half of the maxilla is perforated by an incisive 

 canal through which the nose communicates with the mouth. The 

 maxilla also contains a large sinus, the antrum of Highmore, which opens 

 into the narial passage. 



The mandible or lower jaw consists of a body and two rami, which 

 extend upwards nearly at right angles with the body. That portion of the 

 jaw in which the teeth are set is the alveolar limbus. One of the dis- 

 tinguishing characteristics of the mandible of modern man, Homo sapiens, 

 is the presence of a chin or mentum, which is lacking in most fossil types. 

 Near the chin on the side of the jaw is a mental foramen through which 

 nerves and blood vessels penetrate the jaw. See Fig. 189. 



Each flattened ramus of the mandible has two prominent processes, 

 the condyloid and coronoid. The head, capitulum, of the condyloid fits 

 into the mandibular fossa of the temporal bone to form the articulation of 

 the jaw. 



The hyoid is a small, flattened, U-shaped bone located in the throat 

 between the larynx and the base of the tongue. Like the mandible, 

 the hyoid consists of a body, corpus, and two paired "horns." The 

 anterior horns are much smaller than the posterior and frequently remain 

 cartilaginous. Each lesser horn is connected by a stylohyoid ligament 

 with the styloid process of the temporal bone. 



