260 ESSENTIALS OF ZOOLOGY 



to the sternum, thus forming with their neighbours of the 

 other side complete arches around the body. Such are called 

 sternal ribs. The cartilage portion of the eighth rib fuses with 

 that of the seventh, and the ninth similarly with the eighth. 

 These are termed asternal. The last three (or four sometimes) 

 terminate freely, and such are called free ribs. Similar con- 

 ditions prevail in other mammals, but as in the horse, for 

 example, free ribs may be absent, all the ribs being attached 

 to one another in succession or to the sternum. The number 

 of thoracic vertebrae corresponds to the number of ribs. 

 Dorsally the ribs are articulated to the vertebral column by 

 the head and the tubercle, and, as has already been noted, the 

 centra of the vertebrae provide half-facets for the head and the 

 transverse processes complete facets for the articular tubercle. 

 This is the typical condition, but it is liable to be departed 

 from, especially posteriorly. 



In the rabbit the sternum consists of seven elements or 

 sternebrae. The first extends forward in front of the first 

 pair of ribs as the manubrium or presternum, and the last 

 behind the last pair of ribs expands into a flattened cartilage, 

 the xiphisternum or metasternum. The intervening five 

 elements constitute the mesosternum. It will be found to 

 vary in different mammals with regard to its ossification and 

 the number of its elements. 



The thorax is thus enclosed by a framework. The ribs are 

 articulated at both ends, joints being provided which admit of 

 hinge action. From the oblique position of the ribs it will be 

 understood that the width of the thorax is increased when 

 the muscles pull them forwards, and is decreased again after 

 the action of the muscles has ceased. This takes place in 

 respiration. 



The skull is introduced during development as a pair of 

 parachordals and a pair of trabeculae. The former fuse, 

 enclosing the head part of the notochord to form the basilar 

 plate ; and the latter fuse with the basilar plate and anteriorly 

 to form the ethmoid. The chondrification of the connective 

 tissue around the brain proceeds so as to surround the medulla 

 in the formation of the occipital region, and only a slight distance 

 up each side of the brain in front of this region. The auditory 



