HORSE. 579 



tubes radiating from the cells. These are more usually elliptical 

 than in the thicker central cement, their long axis being parallel 

 with the borders of the cement ; they are most abundant next the 

 enamel and rarely encroach upon the clear peripheral border 

 of the cement. The exterior coronal cement of the molars (PL 137, 

 c,) is as richly permeated by vascular canals (v) as is the central 

 cement of the incisor. 



The enamel-fibres of the Horse's incisor are very slender, not 

 exceeding twice the diameter of the dentinal tubes : they extend 

 with a gentle sigmoid curve through the entire thickness of the 

 layer ; contiguous fibres curving in opposite directions. The 

 peripheral border, or that next the cement, is everywhere indented 

 with hemispherical pits from ^Joth to jith of an inch in diameter, 

 four to six of the radiated cells of the cement being often clustered 

 together in the larger depressions. The inner or dentinal border 

 of enamel is nearly even and straight ; here are seen the short 

 cracks or fissures extending into the enamel. The fibres are rather 

 more wavy in the thicker enamel of the molar teeth (PI. 137, b) 



If the enamel is viewed in sufficiently thin sections, it is free 

 from those wavy dusky markings which are produced by the more 

 tortuous fibres of the human enamel ; and I have been unable to 

 distinguish any transverse striae in the fine fibres of that tissue in 

 the Horse : the appearance of such is given by thicker sections of 

 the enamel-fibres taken obliquely across them, and is produced by 

 the cut ends of the fibres. 



I may here briefly describe a very rare diseased state, which I 

 have met with in a fossil molar tooth of a large-sized Horse, 

 from the tertiary formations near Cromer. The tooth, which was 

 from the lower jaw, with a grinding surface measuring one inch 

 five lines in long (antero posterior) diameter, and eight lines in 

 short (transverse) diameter, presented a swelling of one lobe, near 

 the base of the implanted part of the tooth. To ascertain the nature 

 and cause of this enlargement, I divided it transversely, and exposed 

 a nearly spherical cavity, large enough to contain a pistol-ball, with 

 a smooth inner surface. The parieties of this cavity, composed 

 of dentine and enamel of the natural structure, were from one to 



p p 2 



