DEVELOPMENT OF THE TEETH. Ill 



oped ; each is partially enclosed in a sac, and receives branches 

 from the vessels and nerves which run along the bottom of the 

 groove. At the fourth month, the enveloping sac is thick in its 

 texture, and consists of two layers, which are easily separated 

 after a short maceration. Both of these layers, Fox and T. Bell 

 have proved, by their injections, to be vascular:* laying loosely 

 within this double sac is the gelatinous vascular pulp itself, 

 covered by an extremely thin, delicate vascular membrane, (to 

 which it is closely united by vessels,) which secretes the bony 

 part of the tooth, and is a sort of internal periosteum.f The 

 pulp and its membrane receive their vascular and nervous fila- 

 ments from the proper dental vessels and nerves, which run 

 along the groove in the jaw. The double saccular membrane 

 receives its vessels and nerves solely from the gums ; and the 

 only attachment between this and the membrane of the pulp, is 

 near the base of the latter, where the dental vessels enter it. 

 The sac is closely united to the gum, hence if we tear away the 

 gum that covers the jaws, we necessarily bring with it the entire 

 structure of the germ. 



If at this period, the fourth month, we open the germ, w,e 

 find the pulp presenting exactly the size and shape of the body 

 of the teeth first cut, (incisors) and that its membrane has 

 already commenced the deposit of the bony tip. 

 At birth, ossification will be found to have commenced on all 

 the pulps of the temporary teeth, (the body of the incisors 

 being nearly completed,) and on each of those of the anterior 

 permanent grinders. The commencement of ossification is by 

 three points in the incisors, which form their serrated edges 

 as seen on their first development, by a single point for the 

 canine, two for the bicuspide, and three, four, or five on the 



* Hunter declared, that the external is soft and spongy, without any vessels; 

 the other much firmer, " and extremely vascular." Blake on the contrary 

 asserts, that the external is spongy and full of vessels, the internal one is more 

 tender and delicate, and seems to contain no vessels capable of containing red 

 blood. 



f This membrane is called by Bell the proper membrane of the pulp, and was 

 conjectured by Blake, with much probability, to be a "propagation of the peri- 

 osteum of the jaw." Blake on the Teeth, p. 8. 



