THE RABBIT 427 



ninth are connected with the ribs in front of them. The 

 last three pairs have no sternal portions and no tubercula. 

 The breastbone or sternum is a long, narrow rod, divided 

 into segments, and lying in the mid-ventral line of the thorax. 

 The first segment is the manubrium. It is the largest and is 

 flattened from side to side. Behind it come four segments 

 of equal size, then a Very short segment, and finally the 

 xiphoid process or xiphisternum, a long, slender rod, which 

 bears behind' a horizontal plate of cartilage. The ribs of 

 the first pair articulate with the sides of the manubrium, 

 and the succeeding six pairs between the segments. 



The skull x contains the same regions that we have met 

 with in the frog and dogfish, but it consists 

 practically entirely of bones, which meet one 

 another by jagged sutures. 



The cranium or brain-case proper is relatively short, lies almost 

 wholly behind the orbits, and is not in a line with the facial region, 

 which is bent downwards at an angle of 60° upon it. Its bones are 

 arranged in a series of three rings. ( I ) The hinder or occipital ring 

 consists of four cartilage bones (p. 31). The basioccipital is a flat 

 bone which forms the floor of the ring, including the lower edge of the 

 foramen magnum and a small part of each occipital condyle. The 

 exoccipitak make the sides of the ring, bounding the foramen laterally 

 and forming the greater part of the condyles. The supraoccipital is a 

 large, median bone which roofs the occipital ring. (2) In the middle or 

 parietal ring there are both cartilage and membrane bones. It abuts on 

 the occipital ring above and below, but at the sides is separated from it 

 by the auditory capsules and squamosal bone. The floor of the cranium 

 in this region is formed by a cartilage bone known as the basisphenoid, 

 which lies in front of the basioccipital. It is triangular with the apex 

 truncated and placed forwards, and upon its upper surface is a hollow, 

 known as the sella turcica, which lodges the pituitary body. The 

 alisphenoids are a pair of irregular cartilage bones which lie at the sides 

 of the basisphenoid and form the lower part of the lateral wall of the 

 cranium. The parietals are two large, square membrane bones upon 

 the roof of the cranium, separated at the sides from the alisphenoids by 

 the squamosals. The parietals meet in the middle line. Behind there 

 is wedged in between them and the supraoccipital a small median 

 interparietal. (3) The foremost or frontal ring contains a narrow 

 median ventral cartilage bone known as the presphenoid, which lies in 



1 The skull of the dog is in some respects a more suitable example 

 than that of the rabbit for the preliminary study of a mammalian skull. 

 Good accounts of it may be found in Flower's Osteology of the Mam- 

 malia, Reynold's Vertebrate Skeleton, and other works. (See also 

 Figs. 348. 349-) 



