THE SKELETON 57 



side of the median line from the adjacent and originally cartila- 

 ginous supra-occipital, it may perhaps have existed in the ancestors 

 of man as an independent bone. 1 



The interparietals first appear in Mammals, but among the 

 higher forms they are seen in a state of apparent degeneration, as 

 would appear from their great variability in occurrence, form, and 

 detailed relationships. They may, for example, remain either 

 partly or wholly isolated ; they may be either single, bilaterally 

 symmetrical, or asymmetrical, or may be represented by but one 

 lateral bone. 



Other inconstant ossific nuclei of this region are the prsein- 

 terparietalia. These may remain partly or wholly isolated, and 

 show in form and position variations similar to those above 

 described for the interparietals. The possible combinations 'of 

 these anomalous bones cannot be discussed here (cf. Fig. 38). 



The morphology of the prseinterparietals is not clear, and it 

 is by no means unlikely that, like the ossa Wormiana (o. suturaria), 

 they fall under the category of accessory ossicles. The problem is 

 rendered still more difficult by the fact that, so far as is known, 

 they are constantly present only in the Horses, while in other 

 Mammals they are of mere sporadic occurrence. In Man, as 

 compared with the latter, they appear comparatively frequently 

 (i.e. 1 per cent). Equally uncertain is the morphology of the 

 os fronto-parietale [os antiepilepticum of the ancients], a bone 

 which occurs very rarely in Man, in the neighbourhood of the 

 fronto-parietal suture. This bone, which is more often found in 

 the Cebidse among Monkeys, and less frequently in Eodents, may 

 be sometimes paired. 



An atavistic significance may be probably attached to a 

 bony process which occasionally appears in Man, behind and 

 externally to the jugular foramen, and into which the rectus 

 capitis lateralis muscle is inserted. This corresponds with the 

 par-occipital or paramastoid processus of many Mammals, which 

 attains its strongest development in Ungulates and Eodents. 



There is one more point worth consideration in the occipital 

 region, i.e. the median portion of the linea nuchse superior. 2 

 A bony ridge (torus occipitalis), stretching at times as far as the 

 linea nuchae suprema, occasionally develops here. According to 



1 "Welcker regards all the larger bones which are occasionally intercalated in 

 the lambdoidal suture as fragments of the os Incae. 



2 It is difficult to decide whether the furrow or pit (fossette verraienne, 

 Albrecht), sometimes formed for the reception of the vermis cerebelli, has any 

 phylogenetic significance. 



