262 



MANUAL OF HISTOLOGY. 



from within outwards, usually three, or sometimes only two, undulating 

 curves (known as the lines of Schreger), and within these again a number 

 of very small jagged or spiral bends, of which about two hundred may 

 be seen in the length of a line (Retzim). 



Like the canaliculi of bone, those of 

 dentine (fig. 252, e) are observed to 

 divide over and over again, and to com- 

 municate through their branches ; though 

 in other respects they differ, owing to 

 their more regular course. 



In the internal portions of the dental 

 tissue a number of divisions take place 

 at acute angles, and in rapid succession, 

 with decrease in the size of the branches. 

 This becomes more rare externally, gain- 

 ing again in frequency in the most super- 

 ficial portions. Thus from one canal a 

 whole system may be produced. 



We encounter further, in many cases, 

 intercommunications between adjacent 

 canaliculi by means of oblique branches 

 (c). This may lead eventually to 

 the formation of a regular network in 

 the external portion of the tissue (fig. 

 253). Here some of the canaliculi join 

 in loops (fig. 252, c), whilst others sink 

 down into the cavities of a granular layer 

 situated at that part (&), and a third set 

 advances beyond the limits of the den- 

 tine into the cement (fig. 252, a), or 

 perhaps (1) into the enamel (fig. 253, c). We will meet with these 

 again. Internally, this system of canals terminates by free openings in 

 the cavity of the tooth. 



The ground-substance of dentine, finally, is a 

 homogeneous substance which may be split into 

 bands artificially after maceration. The direction in 

 which this cleavage takes place is determined by 

 the course of the canals. 



In addition to these elementary and essential 

 features of the tissue in question may be added some 

 of minor significance. For instance, a certain system 

 of irregular cavities, of extremely variable size, named 

 by Czermak " interglobular spaces" (fig. 152, b), 

 exists normally in this tissue, the interstices between 

 the projections of a number of more or less spheroidal 

 masses aggregated in the ground-substance, known 

 as " dentine globules." The cavities in question are 

 found, very numerous and small, principally under the cement covering, 

 or crusta petrosa, of the root. They here enter into the construction of 

 the so-called granular layer of Tomes, and may be confounded with 

 lacunae, more especially as they receive into them the terminations of 

 canaliculi. These interstices, however, contain no air during life but 

 a soft organic mass. 



Fig. 252. External portion of human den- 

 tine d, with coating of cement a; at b, the 

 granular or Tomes' layer of the first of 

 these, with interglobular spaces; c and e, 

 canaliculi 



Fig. 253. External por- 

 tion of dentine, rf, from 

 the crown of a tooth, 

 with its layer of enamel 

 fr. a, enamel cuticle; c, 

 interstices filled with air. 



