ON THE THEORY OF THE VERTEBRATE SKULL 58 1 



The distal, larger part of the haemal semi-arc is commonly dis- 

 tinguished from its proximal smaller part, by the conversion of its 

 ■cartilage into osseous or other tissue, and thus the semi-arc becomes 

 separated into a rib and an articular surface or process, for the head 

 •of that rib, to which last the term Parapophysis may be conveniently 

 restricted. 



In the dorsal vertebra of many Vertebrata, the neural semi-arc 

 rsends out a process, the Diapophysis, which is eventually met by a 

 ■corresponding outgrowth of the rib, its so-called tubercle, and the two 

 .become firmly connected together. 



When ossification occurs, it is a very general, if not invariable rule, 

 that an annular deposit around the notochord takes place in the 

 ■centrum. I term this the Diaphysis of the vertebra. In some fishes 

 a distinct centre of ossification appears in each paraphysial region, 

 and this may be termed the Paraphysis of the vertebra. 



In mammals each end of the vertebra ossifies from a distinct 

 point, and constitutes a central Epiphysis of the vertebra ; and in 

 many Vertebrata a part of the under surface of a centrum ossifies 

 separately as a distinct Hypophysis. It is another very general, if not 

 invariable rule, that a distinct centre of ossification appears in, or on, 

 each neural semi-arc or Neur apophysis, and passes upwards, into the 

 spine or Metaneurapophysis ; downwards, to unite sooner or later 

 with the diaphysis, or diaphysis and paraphysis ; and outwards into 

 the diapophysis. 



It is doubtful whether the paraphysis appears as a distinct osseous 

 element in any Vertebrata above the class of fishes, in very few of 

 which even, is it distinguishable in the adult state. Consequently in 

 the higher Vertebrata the paraphysial region is ossified, either from 

 the diaphysis or from the neurapophysis, or from both ; and a suture 

 exists for a longer or shorter time at the point of junction of the 

 neural and central ossifications. I will term this the Neurocentral 

 suture. Its position is no certain or constant indication of the nature 

 of the parts above or below it, for it may vary in the same vertebral 

 column from the base of the neurapophysis, to the junction of the 

 paraphysial with the diaphysial region of the centrum. 



The number of the centres of ossification in each distal portion of 

 the haemal semi-arc may vary greatly ; the uppermost is called a 

 Pleurapophysis, the lower, Hcemapopkyses and Met-hmnapophyses. 



Besides these primary centres of ossification of a vertebra, there 

 are others of less constancy. Thus the ends of the metaneurapophyses, 

 diapophyses, and zygapophyses in many Mammalia are ossified 

 from distinct centres ; and in the caudal region of many of the higher 



