THE RABBIT. 77 



a series of tarsals, forming the tarsus. Of these the proximal 

 row consists of two the preaxial one probably represents 

 both tibiale and intermedium fused : it is rod-like and 

 called the astragalus. The postaxial (fibulare) is still 

 more elongated than the astragalus and projects back into 

 a lever-head (like the olecranon of the ulna) which forms 

 the heel : this bone is commonly called calcaneum.* 



There is a centrale, and only three distinct distalia : the 

 preaxial distale is fused with the second metatarsal (the 

 rest of the great toe or hallux being absent in the rabbit), 

 and the fourth and fifth distalia are fused, just as in the 

 carpus. Each of the four fcoes has a metatarsal and 

 three phalanges. It is often convenient in comparing 

 different animals to express the number of digits and of the 

 phalanges in them by a brief formula. This digital 

 formula in the case of the rabbit would be 



2.3.3.3.3. 0.3.3.3.3. 



14. Sesamoid Bones. Besides the bones of the limb-skeleton 

 already described, which are all cartilage-bones, there are a few 

 others, which are neither cartilage-bones nor membrane-bones in the 

 ordinary sense. These sesamold bones are ossifications in the tendon 

 of a muscle at a point where it has to run over a projecting surface. 

 The best known of these is the knee-cap or patella, developed in the 

 tendon of one of the extensor muscles of the leg. 



15. The Skull. The mammalian skull will be better 

 understood after the study of that of some lower vertebrate. 

 We shall describe its main features now, but their meaning 

 will become much clearer later on. Our figures are of the 

 dog (Oanis familiaris). In section (fig. 47), we perceive a 

 brain-case (cranium) opening behind by a large aperture, 

 the foramen magnum. In front of this is an extensive 

 passage, the nasal passage, which is divided from the mouth 

 by a bony floor, the palate, and which opens into the 

 pharynx behind at the posterior nares, and to the exterior 

 by the anterior or external nares. Its anterior portion is 



* The terms astragalus and calcaneum are taken from human 

 anatomy ; tibiale and Jibulare are the terms of comparative anatomy. 

 Most of the other carpals and tarsals also have human-anatomy 

 names, but it seems unnecessary to give them here. 



