OSSEOUS SYSTEM. 189 



the sphenoid, and are united in front with the palatal bones, and 

 posteriorly and inferiorly with the articular portion of the temporal. 

 Occasionally, as in Pleuronectes, they are divided into two pieces, 

 an internal and external alar lamina. As constituent elements of 

 the temporal bone, we regard with more or less justice a remarkable 

 number of ossicles, which may be resolved into two principal divis- 

 ions, belonging to the cranial and articular portion of that bone. 

 The cranial division consists always of three bones, which, inter- 

 calated between the already described pieces of the occipital and 

 sphenoid bones, are united with these, as well as the parietals and 

 frontals, by suture. The petrous bone is of larger size, disc-shaped, 

 and is situated most deeply, being interposed between the body and 

 inferior occipital bone, and also the great wings of the sphenoid ; it 

 rests upon the body of the latter bone, and is perforated by a large 

 opening for a branch, the opercular, of the fifth pair of nerves. In 

 the direction upward and backward is placed the mastoid bone, 

 which by some has been taken for the squamous element. We 

 may regard, however, as the squamous portion of the temporal, a 

 bone which rests in front of the mastoid, above and upon the pe- 

 trous ; but if this analogy will not hold good, it must be viewed as 

 a particular scale-like bone, comparable to that which occurs in the 

 Reptilia, and be called the posterior frontal. Between this portion 

 of the skull and the lower jaw a number of bones, amounting to 

 five at the utmost, or four, three, or only two in number, are intro- 

 duced ; the most anterior of these articulates with the lower jaw, 

 and constitutes the articular portion of the temporal, which, in Rep- 

 tiles and Birds, is reduced to the single quadratal bone. The first, 

 the uppermost and most posterior bone, is always the largest in 

 size ; it forms the superior articular bone, and unites itself by means 

 of a mostly moveable process with a corresponding depression in the 

 mastoid piece and squamous element of the temporal bone ; behind 

 and above we meet with a condyle upon it for articulation with the 

 operculum. In front of, and leading somewhat downward from, 

 the superior articular bone, lies the great opercular, a. flat and very 

 thin bone, beneath which again is placed the narrow hamular ossiclCj 

 and against this the inferior articular, which articulates with the 

 lower jaw, abuts in the direction forward and downward. There 

 is found sometimes a fifth, smaller and flatter bone, situated between 

 the others. The above-mentioned bones are partly united together 

 by squamous suture, partly by fibro-cartilage, and concur to form a 

 bony wall, abutting posteriorly against the prazoperculum, which 



