THE SKELETAL SYSTEM 173 



of the articular element and a corresponding enlargement of the dentary. 

 The relation of the two bones is such that the lower jaw at one time may 

 have been hinged by both. The enlargement of the dentary and the 

 diminution of the articular set free the latter to pass into the tympanic 

 cavity and join the chain of ear bones 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. 164) 



The evidence, therefore, compels us to beUeve 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 two pairs 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. 



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

 extend upwards nearly at right angles with the body. 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. 

 See Fig. 158. 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 carti- 

 laginous. Each lesser horn is connected by a stylohyoid ligament with the 

 styloid process of the temporal bone. 



Included in the visceral skeleton of man are the cartilages of the larynx 

 or "voice-box." The largest and most prominent of the laryngeal 

 cartilages is the thjrroid or shield-shaped cartilage. The right and left 

 halves of this cartilage stand at approximately right angles to one another. 

 From its dorsal borders extend paired superior and inferior horns. 



The cartilage immediately below the thyroid has the shape of a seal- 

 ring and consequently is called the cricoid cartilage . The cricoid expands 



