204 MANUAL OF HISTOLOGY. 



indistinctly fibrous, and devoid of elastic elements. ^ The pulp is further 

 rich in nerves and very vascular, almost presenting in transverse section 

 the appearance of a cavernous tissue. The small arterial stem which pene- 

 trates into its substance splits up into several branches, which advance 

 through the latter, forming in the crown of the tooth numerous capillary 

 loops through which transitions to veins having a similar course back 

 a<*ain' takes place. The nutrition of the tooth is presided over by these 

 vessels. The nerves will be referred to in a subsequent section. To 

 them is due the great sensitiveness of the tooth, which, as is well known, 

 may increase to intense painfulness at times. The external surface of 

 the pulp is covered by a laminated stratum of narrow cylindrical cells, 

 0-0452-0-0902 mm. in depth, resembling epithelium. These elements, 

 0-020-0*030 mm. in length, contain an elongated nucleus. They 

 are connected in the first place with one another by means of their 

 ramifications, and in the next with the deeper-lying cellular ele- 

 ments : finally, they send off soft delicate processes, single or multiple, 

 externally. 



The "dentine cells," or, as they have been more recently and better 

 named, the " odontoblasts " (Waldeyer) (fig. 255, I), have been long 

 known ; but attention has only been directed gradually to their relations 

 to the dental tissue. 



It used to be thought that the system of canaliculi was nothing but 

 a series of canals possessing no formed con- 

 tents, and only filled with a nutritive fluid 

 (Leasing). Indeed, dentine appeared to pre- 

 sent one of the 'most beautiful examples of a 

 system of vessels for plasma in the whole 

 group of connective-substances. 



But Tomes' discoveries, confirmed by the 

 observations of Beale, Koelliker, Neumann, 

 Frey, Waldeyer, Hertz, and Soil, showed the 

 Fig. 255. TWO dentine ceils, 6, errorieousness of this older view. 



traversing with their processes ^ TT ... , , 



a portion of the canaliculi at a, We may easily convince ourselves, namely, 



that the odontoblasts protrude those of their 

 processes already mentioned as directed out- 

 wards into the so-called canaliculi of the tooth 

 (fig. 255, ), probably traversing the latter with their ramifications in 

 their whole length ; at least, they may be still seen in the crown of the 

 adult tooth. It would appear also as though these fibres of Tomes, or 

 " dental fibres," filled up the whole lumen of the passages. 



It has been supposed that the structures resembling canaliculi, isolated 

 by means of maceration, are nothing but these ramifications of the den- 

 tine cells. This, however, is not correct ; for even after processes which 

 must have destroyed all the softer parts of the tooth, after the most 

 active decomposition, canaliculi endowed with a special wall may be laid 

 bare (Neumann). 



This wall can just as little be looked upon as the calcified membrane 

 of dentine cells and their processes as in the corresponding elements of 

 bone. It is here also, as in bone, a modified bounding layer of the 

 ground-substance, so that we are correct in speaking of " dental sheaths " 

 (Neumann, Waldeyer, Boll). 



Tomes' view of the matter is of great interest : he refers the great 

 sensitiveness of the dentine to the soft fibres of these cells. We shall 



