PROCURING OBJECTS. 99 



matter. These should be mounted in fluid, the others dry, or 

 in balsam. 



A tooth consists of three distinct structures, the relative pro- 

 portions and arrangement of which constitute the chief differ- 

 ences in the teeth of various animals. 1. Enamel. This is 

 crystallized phosphate of lime, deposited in the form of long 

 prisms each about 39*00^ ^ an ^ ncn i diameter, produced in 

 animal cells which are almost obliterated when the tooth is 

 fully formed. In human teeth a coating of enamel is formed 

 over the crown of each. In the teeth of some animals the 

 enamel is disposed in vertical layers among the other struc- 

 tures of the tooth. This is especially the case with the grind- 

 ing teeth of large herbivorous animals. 2. Dentine, or Ivory. 

 This forms the principal substance of which the teeth are com- 

 posed. The amount of animal gelatine in it is often very 

 considerable. The earthy matter is usually deposited in the 

 form of fine branching cylindrical tubuli, radiating from the 

 centre of the tooth. These tubules have been successfully 

 injected with coloring matter by soaking the tooth in a solution 

 of Saunder's wood, &c. On the ends of the dentine tubuli are 

 placed the ends of the enamel prisms, in the human tooth. 

 Dentine is now established by Professor Owen as an ossification 

 of the pulp of the tooth. 3. The bone or Cementum, of 

 teeth, is composed of a mass of earthy matter and cartilage, 

 having minute cavities or bone-corpuscles and calcigerous 

 canals. 



Sometimes a vertical section is made of a tooth in situ, ex- 

 hibiting a section of the jaw with its cavities for the nerves 

 and vessels, as also the manner in which the alveolar process 

 which forms the socket is constructed. Both vertical and trans- 

 verse sections should be made. 



SKIN. The skin is supplied with a very rich capillary 

 network ; and also provided with two or more sets of glands, 

 one for secreting the perspiratory fluid, the other an unctuous 



