STRUCTURE OF THE CEMENT. 475 



this substance could always be referred to imperfect purification 

 of the membrane from enamel or dentine in contact with it ; 

 so that it is questionable whether it undergoes any calcification. 

 Kollmann (67a) has recently admitted this, but offers no proof. 



CEMENT (Zahn-kitt, osteoid substance, cementum, tortex os- 

 seus, crusta fibrosa). The cement is a true bony structure 

 essentially belonging to the periosteum of the alveolus, and in 

 man and many of the vertebrates forms a thin investment to 

 the fangs of the teeth. Intimately connected with the dentine, 

 it commences as a delicate covering at the neck of the tooth, 

 where the enamel ceases, and is thickest at the apices of the 

 roots and in the depressions between the roots of the molar 

 and bicuspid teeth. In the folded enamel and compound teeth 

 the cement penetrates deeply in the form of a moderately thick 

 layer between the projections of the crown, or serves as a 

 connecting substance to the several toothlets ; it is therefore 

 situated for the most part external to all the other constituents 

 of the tooth. The Pachydermata and others have also a special 

 covering of cement, investing the whole crown of the tooth as 

 a secondary formation (crown cement). 



Both in its chemical and microscopical characters, cement is 

 closely allied to bone. The lacunae are for the most part large, 

 and possess an enormous number of very long canaliculi, es- 

 pecially in the Cetacea. When the cement is extremely thin, 

 however, they may be entirely absent, and it then presents 

 on section a perfectly homogeneous and vitreous appear- 

 ance. A similarly very hard lamella, destitute of lacunae, 

 occurs also in the outermost portion of the thicker layers of 

 cement. Haversian canals, which sometimes open into the pulp 

 cavity (Salter, 58) are found when the cement is thick, though 

 it is rare to find any lamellated arrangement of the matrix. 



Kolliker (58) has described peculiar cavities in the cement, which he 

 considers to result from pathological processes. Skarpey's fibres also 

 occur, and I have found the cement of the dog to be that best adapted 

 to show them. The thick capsule-like investments surrounding one 

 or several lacuna, first noticed by Gerber (24) in the cement of the 

 horse, are deserving of especial mention. These lacunas, with their 

 thick capsules, can be easily isolated in diluted acids, and may be 



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