THE MICROSCOPE IN ANIMAL HISTOLOGY. 197 



kinds of tissue: the cement, a bony substance, coating the 

 root of the tooth, containing bone-cells and canaliculi,but 

 no Haversian canals, the pulp in the central cavity of the 

 tooth serving for the nutrition of the organ, as a large 

 Haversian canal ; the dentine, or ivory, constructed as 

 above described; and the enamel, covering the crown, arid 

 consisting of columns or prisms, often hexagonal, which 

 are the hardest and densest structures of the body (Plate 

 XX, Fig. 152). 



The development of enamel from epithelium has been 

 referred to on page 192. The dental germ corresponds to 

 a papilla of the mucous membrane, and in an early stage 

 is covered by delicate stratified cells the dentine cells, 

 or odontoblasts which produce dentine. Teeth are thus 

 produced abnormally in other situations besides the jaws, 

 as in ovarian cysts, etc. 



Before the development of the 'first, or milk teeth, the 

 rudiments of the permanent teeth exist as a fold or leaf 

 of epithelium springing from the enamel germ. 



II. COMPOUND TISSUES. 



1. Muscle. This is the tissue by which the principal 

 movements of the body are performed. It consists of 

 fibrin, which is endowed with special contractile power. 

 It is of two kinds, the voluntary, pertaining to organs of 

 voluntary motion, and the involuntary, found in situa- 

 tions which are not under the control of volition, as the 

 coats of bloodvessels, alimentary canal, uterus, and blad- 

 der. The fibres of voluntary muscles are marked with 

 transverse striae. Involuntary muscular fibres are smooth, 

 except in a few instances, as the fibres of the heart and 

 some of those in the oesophagus, which are striated. 



The fibres are connected with and invested by connec- 

 tive tissue, and arranged in parallel sets, with vessels and 

 nerves in the intervals, and are attached to the parts they 



