MAMMALIA 278 



layer of the pulp within gives rise to periosteal-like cells, but 

 called odontoblasts, which proceed in forming an internal 

 lining to the enamel cup of dentine or ivory. The tooth thus 

 proceeds in its growth beneath the gum, and consists of an 

 enamel crown within which is the dentine and within it again 

 the pulp, the last becoming narrower from the activities of the 

 odontoblast layer. When it is erupted the pulp cavity is 

 more and more reduced as the dentine becomes thicker, and 

 gradually it shrinks into a small canal, thus providing for the 

 wear of the teeth. During the years of growth it also continues 

 to add to the length of the tooth. In the case of the rabbit, 

 however, the pulp maintains its position and activity in the 

 lower part of the tooth, and growth proceeds during life from 

 the base, the pulp retreating as the tooth is worn at its free end. 

 The teeth are said to grow from a permanent pulp, and this is 

 the reason that in accidents to the teeth the continued growth 

 brings them into a state which may lead to the death of the 

 animal. 



Turning now to the rabbit, the internal structure can be 

 made out by dissection. With the animal fixed to the dissecting 

 board, ventral side upwards, the skin is removed by a median 

 incision and the two flaps reflected. Attention should be 

 directed first to the abdomen, and later to the neck and thorax. 

 The skin is smooth and white on the inner surface. The dermis, 

 as has been noted, is made of connective tissue, and along its 

 ventral aspect the sheet of cutaneous muscle or panniculus 

 carnosus is to be seen, and in the female the mammary glands. 

 It is connected to the muscular body wall generally by con- 

 nective-tissue strands, but mainly in the region of the limbs. 

 The body wall, in addition to the skin, consists of muscle which 

 forms a thin envelope over the abdomen ventrally and laterally. 

 The bay formed by the ribs in front is occupied by the flat 

 cartilaginous xiphisternum. Between this and the pubis is the 

 median band of connective tissue, the linea alba ; and between 

 the ilium and the pubis, Poupart's ligament, which forms an 

 arch through which the nerves and vessels pass to the leg. 

 The external oblique muscle of the abdominal wall runs from 

 above inwards to the linea alba, and a slight incision will 

 demonstrate that it forms a thin sheet, beneath which is 



