180 BIOLOGY 



spinal cord extends into it, but soon passes out through two 

 small openings, on either side, o, as two small filaments. This 

 bone represents the tail found in allied animals (salamanders). 

 The frog has no ribs and the transverse processes end abruptly 

 at a short distance from the centrum. 



The skull. In front the first vertebra is articulated with 

 the skull, and the neural canal is continued into the skull 

 through a large opening, called the foramen magnum (Lat. 

 foramen = hole), fm. Inside the skull is a large cavity hold- 

 ing the brain, the cranial cavity. The skull itself is com- 

 posed of thirty-two bones, rigidly fused together to form a 

 solid structure. These bones, which are shown and named 

 in Figure 88 A and C, may be divided into three groups : 1 . The 

 cranial bones, which form the roof, walls, and floor of the cranial 

 cavity. The floor is made of the basioccipital and the para- 

 sphenoid, ps; the walls are made of the parietals, p, the otic 

 bones, and the exoccipitals, ex; and the roof is made of the 

 supraoccipitals and the frontals, /. 2. The facial bones, which 

 form the face. These are the nasals, na, the premaxillas, pr, 

 and the maxillas, mx, above, and the vomers, vo, below. 3. The 

 branchial (Lat. branchice = gills) skeleton. This part of the 

 skeleton is made primarily of two V-shaped arches, lying below 

 the cranium with the open part of the V above, next to the skull ; 

 but the original relation of the V-shaped arches has become so 

 modified that it is difficult to recognize. The first of the arches 

 is the lower jaw or mandible; Fig. Z>, m. The closed part 

 of this arch is in front where the two halves come together. 

 At the back the two halves spread apart and pass backward 

 to the point where the jaw articulates with the cranium at q. 

 The lower jaw is from this joint held attached to the cranium 

 by two chains of bones. One of them is made of the quadrate 

 (Fig. D, qu), and the squamosal, sq, these two forming what 

 is sometimes called the suspensorium. The other chain is 

 made of two bones lying below the cranium, the pterygoid 

 (Fig C, pi), and the palatine, pa. These are firmly fixed to 



