THE RABBIT. 57 



with very little organic matter) that forms the capping of 

 the teeth. The main portion of the tooth is formed of 

 dentine, a tissue resemblingibone in composition, but differing 

 from it in the fact that the active cells (odontoblasts) form 

 a continuous layer, and deposit their secretion at their outer 

 ends only, so that they never become embedded in their 

 matrix as do osteoblasts. Within the dentine is a " pulp- 

 cavity," resembling the marrow-cavity of bones and 

 containing connective tissue with abundant blood-vessels 

 and nerve-fibres. In the incisor teeth of the rabbit this 

 pulp-cavity persists throughout lif e, so that the odontoblasts 

 continue to receive nourishment and to secrete, and the 

 tooth goes on growing in length throughout life. The 

 cheek-teeth, however (and the incisor teeth of such animals 

 as the dog) have the opening of their pulp- cavity gradually 

 constricted at the base as the tooth grows, and finally closed 

 in, so that the odontoblasts are cut off from their food- 

 supply, and the tooth ceases to grow. 



The third component of teeth is the " cement " at the 

 base : this is true bone. 



The development of teeth will be more conveniently 

 considered at a later stage. 



13. Blood. The student will be surprised to hear 

 blood considered among the "connective tissues." It is, 

 however, strictly a liquid tissue, though the plasma does not 

 agree with the ordinary matrix of a tissue in being entirely 

 a secretion of the corpuscles. The identity of the white 

 blood-corpuscles with connective-tissue corpuscles must have 

 already been realized by the student. The red corpuscles, 

 however, lack a nucleus, and are therefore not cells : in 

 accordance with that deficiency, they have no power of 

 growth or division, and their existence is not indefinite in 

 length, but comes to an end after a short (but unknown) 

 period. Old red corpuscles are constantly being destroyed 

 and new ones formed : the spleen appears to be the region 

 in which the dying ones are retained, and it would seem 

 that the haemoglobin set free from them is carried on to 

 the liver, and there goes to form the bile-pigment bilirubin. 

 New red corpusles are formed mainly in the red marrow 



