Thk Hyoid. ■ 97 



of the horizontal portion with the ramus. The corresponding mental 

 foramen (foramen mentale), through which branches of these structures 

 leave the mandible, is situ.ated on the lateral surface in front of the first 

 premolar. The mandibular foramen is closely connected with a second 

 aperture lying at the ventral end of the sulcus ascendens, directlv behind 

 the last molar, and serving for the transmission of a vein connecting the 

 inferior alveolar and inferior orbital veins. 



The mandibular ramus forms in general an obtuse angle with the 

 horizontal portion. As in other herbivores, the ventral part, distin- 

 guished as the angle, is greatly increased in size at the expense of the 

 condyloid process and to a still greater extent of the coronoid process, 

 the latter being vestigial. In addition to a low pterygoid tuberosity 

 (tuberositas pterygoidea), situated at the posterior projecting point of 

 the angle, the posterior and ventral margins of the angle are excavated 

 on the medial side of the bone, so that they form the boundarv of a pro- 

 nounced, though shallow, pterygoid fossa for the insertion of the ptery- 

 goideus internus muscle. A somewhat similar, but less developed, 

 masseteric fossa occupies the corresponding lateral surface of the angle, 

 its raised ventral margin terminating posteriorly in the masseteric 

 tuberosity (tuberositas masseterica). The articular portion, or head of 

 the mandible is greatly elongated in the anteroposterior direction in 

 accordance with the anteroposterior action of the lower jaw, this feature 

 being one which is of general occurrence in the rodent order, and more 

 fully expressed in the great extension forward and backward of the 

 attachment areas of the muscles of mastication. The connection of the 

 articulating portion with the condyloid process, the so-called neck of the 

 mandible (collum mandibulae), is a thin plate of bone, the anterior and 

 posterior margins of which are barely notched by the anterior and 

 posterior mandibular incisures. Connecting the anterior incisure with 

 the rim of the alveolus of the last cheek-tooth there is a deep groove, 

 the sulcus ascendens, the lateral margin of which is formed by the re- 

 duced coronoid process (processus coronoideus). Its low medial margin 

 is formed by a bony stay which extends to the medial surface of the 

 horizontal portion opposite the last cheek-tooth, and is continued for- 

 ward into the mylohyoid line. The sulcus ascendens lodges in the 

 natural condition the insertion portion of the greatlv reduced temporalis 

 muscle. 



C. THE HYOID APPARATUS. 



The hyoid bone (os hyoideum) (Fig. 36) is a stout, somewhat wedge- 

 shaped bone lying in front of the larynx and between the angles of the 

 mandible. Its ventral portion is connected with the thyreoid cartilage 

 of the larynx by the median hyothyreoid ligament. With its lateral 

 portion are articulated two independent elements, termed the lesser and 

 greater cornua. The lesser cornu (cornu minus) is a small, partly car- 

 tilaginous structure, attached to the anterodorsal angle of the hyoid, 

 and connected through the stylohyoideus minor muscle with the jugular 

 process of the skull. The greater cornu (cornu majus) is a larger element 

 extending obliquely dorsad, and similarly suspended from the jugular 



