TRANSACTIONS OF THE SECTIONS. 17 



and which he is disposed to believe is so characteristic in different animals 

 as to be capable of affording valuable aid in the classification of the animal 

 kingdom. This structure he first observed in a section of a fossil tooth of a 

 rhinoceros, by the aid of a magnifying power of one-tenth of an inch focal 

 distance, with an achromatic condenser of the light. The section presented 

 an appearance of cells or compartments, the form of the cells varying in 

 different animals; the structure also of the fibres of different teeth present- 

 ing an interrupted or baccated appearance ; the size and relative position of 

 these divisions of a fibre differing in various series of animals. In man, each 

 division is of an oval form, and connected [by] their longer axes, which 

 ■ correspond with the course of the fibre. In some species of quadrumana, 

 the fibre appears to consist of two parallel rows of compartments. In the 

 orang outang, the form is rhomboidal ; and in the baboon, they are oval, as 

 in man. The laminated concentric structure of the tusk of the mammoth, 

 the strength of ivory when cut parallel to the long axis of the tusk, and its 

 weakness if cut at right angles, are urged in corroboration of this peculiar 

 structure. The structure of the enamel, as seen in a section parallel to the 

 long axis of a tooth, presents compartments of a semicircular form ; the con- 

 vexity of the semicircle looks upwards towards the free external portion of 

 the tooth. The chemical composition of the enamel has hitherto led to the 

 conclusion of there being only a small portion of animal matter in enamel. 

 From Dr. Thomson's recent analysis it appears that this has been much 

 understated. The pulp is observed to be cellular throughout its internal 

 structure, and this structure is essentially concerned in the development of 

 the ivory, in the production [of] both [the] fibres as well as of the inter- 

 fibrous substance. There exists a great analogy between the internal or pro- 

 ductive surface of the capsule and the external or productive surface of the 

 pulp. The membranous investment of the enamel in human teeth, lately 

 discovered by the author, displays a similar arrangement. The crusta petrosa 

 is provided with a membranous investment. 



" The above is a correct copy of my rough copy. 



(Signed) « G. Lloyd*," 



(C.) — Mr. Nasmyilis Abstract of the Three Papers, sent to Mr. Phillips on 

 the 2Mh February 1840, abbreviated at Mr. Phillips's suggestion, and 

 finally corrected by Mr. Nasmyth in its progress through the Press in May 

 1840. The italics are Mr. NasrnytKs. 



1. The Capsular Investment of the Enamel. — The Crusta Petrosa, said 

 Mr. Nasmyth, had been described after Retzius, Purkinje and Frankel, as a 

 layer external to the ivory of the fangs of the simple and compound teeth of 

 man, and mammalia generally, but as not present in the simple teeth, as a 

 covering to the enamel. Now this latter position he not only controverted, 

 but maintained, in direct opposition to it, that the enamel itself possesses iji all 

 instances a distinct envelope or coating. On the incisors and other simple 

 teeth of many animals he had succeeded in tracing, in this envelope, the 

 corpuscles of Purkinje, analogous to those found in bonef. 



2. Mr. Nasmyth next passed to the consideration of the interjibrous sub- 

 stance of the ivory. Purkinje and Frankel had stated that " the proper den- 



• The wortls within brackets are noticed by Dr. Lloyd as having been accidentally 

 omitted in his original manuscript, and were since supplied by him as being necessary for the 

 sense. 



f In the 8th Vol. of the Med. and Chir. Transactions is a paper by Mr. Nasmyth contain- 

 ing a full account of his discovery of the Capsular Investment of the Enamel. 



1841. . c 



