1885.] OF THE HUMAN SPHENOID BONE. 583 



fuses with the corresponding lingula, with which element the internal 

 pterygoid also has after birth au osseous union ; the space left 

 between tliis triple union of ala, lingula, and pterygoid being 

 occupied by the vidian or great superficial petrosal nerve, a nervous 

 cord of no small morphological significance, as I have previously 

 shown. The external pterygoid plate is simply an ajiophysis from 

 the under surface of the alisphenoid, becoming conspicuous about 

 the third month of intra-uteriue life. As a matter of convenience and 

 ready reference, the dates of appearance of the individual centres and 

 their fusion with one another are here given in a collected form. 



At the eighth week the following centres appear quickly one after 

 the other, in the following order : — 



1. Alisphenoids. 



2. Basisphenoids. 



3. Lingulse sphenoidales. 



4. Internal pterygoids. 



During the third month the ossific points fuse in the following 

 order : — 



1 . Basisphenoidal nuclei coalesce, and the 



2. Lingulse join tiie basisphenoid. 



At the third month the following centres appear : — 



1. Orbito-sphenoid. 



2. Presphenoids. 



At the fourth month the orbito-sphenoids fuse with the presphe- 

 noids and the internal pterygoids join the alisphenoids. At the 

 seventh month the presphenoid and postsphenoid coalesce. At the 

 eighth month the presphenoids fuse together. 



During the first year after birth the alisphenoids bearing the in- 

 ternal pterygoids coalesce with the lingulse, and the so-called 

 sphenoidal tubinals develop. The strip of cartilage which is pro- 

 longed from orbito-sphenoid to the anterior lateral fontanelle (the 

 pterion) now disappears. 



We must now deal with the morphology of the various centres of 

 the sphenoid. 



In the determination of the regions of the skulls in different types 

 we are greatly assisted by the disposition of the cranial nerves, 

 which in the majority of cases serve as fairly reliable guides. 



The optic nerves always embrace the presphenoid, whilst the third 

 division of the fifth cranial nerve usually quits the skull between 

 the most anterior part of the periotic capsule and the alisphenoid, 

 whilst the auditory nerve stands in very definite relationship with 

 the various constituent nuclei of the periotic capsule, in those forms 

 in which this cartilage undergoes ossification, so that as a rule we 

 have no difficulty in distinguishing between the regions. When deal- 

 ing with the individual ossific nuclei the case is very different, it 

 being absolutely necessary to watch every stage of the development to 

 avoid falling into the numerous pitfalls which abound on every side. 



The presphenoid nuclei do not offer very much that is important, 



38* 



