1895.] ^l [Slade. 



This osseous bridge connecting the lateral regions of the cranium with 

 those of the face is often composed of three bones, the malar or jugal in 

 the centre, flanked on either side by the zygomatic process of the squa- 

 mosal and by the malar process of the maxilla. Again, it may be reduced 

 to two, the process of the squamosal and the jugal, or the process of the 

 squamosal and the postorbital process of the frontal. The number of 

 bones present depends upon the advanced or receding position occupied 

 by the orbit, also upon the position held by the articulation of the mandi- 

 ble in relation to the orbital cavity, whether this be above, below, or on a 

 level with the latter. Although the arch in certain cases is imperfect, it 

 can rarely be said to be entirely absent. 



The strength of the jugal arch, the most important factor in its exist- 

 ence, depends upon its line of direction, whether this be straight or curved, 

 and upon the amount and manner of this curvature ; upon the number, 

 size, extent of surface, and mode of union of its component bones. These, 

 in their turn, are correlated with the articulation of the lower jaw, and 

 with the amount of surface presented by the ascending ramus ; with the 

 neighboring fossae, crests and processes ; with the dental series, and neces- 

 sarily with the muscles concerned in mastication, varied as these are in 

 their action. 



The jugal arch, as it exists in the Carnivora, offers an instructive example 

 of the various points to be considered in its morphology. In the tiger, for 

 example, the arch, composed of three bones — the squamosal, malar and 

 maxilla — presents an extraordinary horizontal curvature, thereby vastly 

 increasing its expanse, giving great width to the temporal muscle, which 

 taking its origin from the largely expanded surface of the parietal, and 

 from the occipital-sagittal crest, passes forwards and downwards, to be 

 inserted into the high, wide, oblique, coronoid process of the mandible. 

 This increase in length of the arch, due to the great horizontal curvature, 

 is also seconded by the advanced position of the orbit upon the skull, 

 and by its height above the level of the articulation of the mandible. 



The vertical curvature of the arch, with the convexity above and con- 

 cavity below, denotes increased power of resistance to the strain produced 

 by the muscular fibres of the masseter, which, springing from the under 

 side of the arch, are carried obliquely backwards and downwards to be 

 inserted into the deeply grooved ascending ramus. The action of the 

 pterygoids, which is similar to that of the masseter, is also relatively 

 powerful. The fibres rising from the pterygoid fossae and plates are 

 inserted into the inside of the angular portion of the lower jaw, and into 

 the neck of the condyle. The suture by which the processes of the squa- 

 mosal and jugal are joined, extends very obliquely through a greater por- 

 tion of the arch ; this obliquity imparting much strength to the bony 

 structure, and thereby enabling it to resist the upward pressure. 



The convex surface of the transverse condyle of the mandible, received 

 into the deeply grooved glenoid cavity, forms the hinge-^like articulation 

 fitted for the vertical action of the jaw, and which is necessary for the pre- 



