236 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



raovably articulated at one end by two convex surfaces with the 

 squamosal and pro-otic, while the other presents a transverse 

 articular condyle for the mandible. Upon the ventral surface 

 of the skull an X-shaped structure is formed by four bones, 

 pterygoids behind and palatines in front. The pterygoid arti- 

 culates behind with a facet on the quadrate, in front with a facet 

 on the side of the parasphenoid, at its base. When the facial 

 part of the skull is moved up and down, the pterygoids readily 

 slide forwards and backwards on these smooth facets. The 

 palatine is a flattened rod, broadest behind, where it meets its 

 fellow and the pterygoid, and tapering to a point in front, where 

 it fuses with the maxillo-palatine process. 



The lower jaw or mandible consists of two almost straight 

 halves (rami), each of which articulates behind with the corre- 

 sponding quadrate, and unites with its fellow in front to form a 

 median symphysis. The axis of the ramus is formed by 

 Meckel's cartilage, which persists, more or less, throughout life, 

 and is ossified proximally into the articular, which is jointed on 

 to the quadrate. Angular and supra-angular elements ensheath 

 the ramus below and above in its proximal half, the latter being 

 produced above into a small coronoid process, while the distal half 

 is covered ventrally and externally by the dentary, internally by 

 the splenial. 



Hyoid and First Branchial Arches. The proximal end of the 

 hyoid arch is formed by the columella, which helps to transmit 

 sound-waves to the internal organs of hearing. Its inner end is 

 bony and expanded into an oval plate fitting into the fenestra 

 ovalis, while its outer cartilaginous end is firmly fixed to the 

 inner side of the tympanic membrane. The " hyoid apparatus " 

 consists of the body of the hyoid, supporting the base of the 

 tongue, and anterior and posterior cornua. The body is composed 

 of three median pieces : (a) A cartilaginous glosso-liyal, shaped 

 like an arrowhead, from which the short anterior cornua also of 

 cartilage, diverge backwards; (6) a short bony basi-hyal; and 

 (c) a styliform bony basi-branchial. Between (b) and (c) the 

 slender three-jointed posterior cornua diverge backwards and 

 upwards. 



The basi-branchial and posterior cornua represent the first branchial 

 arches. 



