MORPHOLOGY OF THE SKILL 89 



l)leiidcd, and the roots coalesce and taper to a cone. Tlie apex is often licnl. 

 The characters of this tooth are very varial)le. 



The third lower molar has a larger crown than the corresponding tooth in the 

 uii])er set; it resembles the adjacent molars and has usually tive cusps. It has two 

 roots which may be confluent. 



The L'eJdfidiis of tlie Croicns of the Upper dud Luircr Teeth to one anotJier. — In a 

 normal condition the upper teeth form a larger arch than tlie lower. The ujjjjcr 

 incisors and canines close in front of tlie lower; occasionally they fall upon, but 

 rarely fall liehind them. The laljial tul)ercles of the liicuspids and molars of the 

 lower jaw are received in the depressions between the la])ial and lingual tul)ereles 

 of the upper set of teeth; hence the labial tubercles of the upper overlap the 

 corresponding tubercles in the lower teeth. 



In consequence of the difference in width of the crowns of the upper and lower 

 incisors, it liappens that in closure of the mouth each tooth impinges upon two 

 teeth. 



The milk teeth. — These are smaller in numljcr and size than the teeth of the 

 permanent set. The formula is: — 



di - dc i- dui - = 20. 

 2 12 



The teniporarv teetli are smaller than their successors; the enamel of the crown 

 terminates in a thick edge; and the tubercles on the crowns of the molars are less 

 regular and pronounced. 



The incisors are similar to those of the permanent set, l)Ut tlie canines have 

 shorter and Ijroader crowns than their successors. 



TILE MORFLLOLOGY OF TILE SKULL 



la Man the skull during development passes through three stages. At first the brain 

 vo.>icles are enclosed in a sac of indifferent tissue which ultimately becomes tough and fibrous. 

 Tliis is the membranous cranium ; a portion of it is represented in the adult by tlie chua mater, 

 the remainder is converted into the membrane-bones. Gradually the sides and base of tlie 

 membranous cranium become cartilaginous : in due C(jurse osseous tissue appears in themem- 

 branous tracts, and later in the cartilage. Eventually an osseous box is formed, consisting of 

 membrane-bones and cartilage bones intricately interwoven. 



A study of the skull in the chondral stage is very instructive. It consists of two parts : (1) 

 The skull jiroper ; and (2) the ai)])endicular elements. 



The skull ]iroper consists of three regions : — 



The basi-cranial or notochordal region, which ultimately gives rise to the chief jKirts of the 

 occipital lione and a part (^f the sphenoid. 



Anterior to tliis is the trabecular region, from which the remainder of the sphenoid is subse- 

 • lueiitly developed. 



The most anterior portion is the ethmo-vomerine region, from which the nasal septum and 

 its associated cartilages arise. Wedged in on each side between the basi-cranial and trabecular 

 regions is tlie complicated periotic capsule. 



The appendicular elements of the cranium are a number of cartilaginous rods, which undergo 

 a remarkable inetaniori>liosis, and, in the adult, are represented by the ear-bones, the styloid 

 process, and the hyoid. 



The chondro-cranium at the third month presents the following parts. Seen fi-om above, 

 the cartilage extends from the cranial base to a spot midway between the base and the vertex, 

 shading off indefinitely on the membranous wall. The conspicuous oval masses on each side are 

 the periotic cartilages, in which the floccular fossae are consiiicuous objects. Each ]ieri(itic 

 cartilage is joined to the sidienoiil by a strip, termed the sphenotic cartilage, which usually 

 persists in the adult skull. At this date the cartilage for the orbito-sphenoid (the so-called lesser 

 wing) is co-extensive with the ali-sphenoid, and forms i^ait of the lateral wall of the skull. J he 

 snout-like appearance of the anterior part of the skull is caused by the fronto-nasal iilate. On 

 each side of the ethmo-vomerine plate near its anterior terniinatidii there are tw(t small concave 

 pieces of cartilage for Jacobson's organ. They are sometimes referred to as the plougiisliare 

 cartilages owing to their shape. 



