58 VERTEBRATE SKELETON 



slender bars between each two primary gill-clefts, but it is difficult 

 to compare these with any parts of the vertebrate skull. 



In lower Vertebrates the skull is readily separable into two very 

 distinct parts/ a cranium consisting of the case containing the brain, 

 and the capsules around the organs of special sense — ^nose, eyes and 

 ears. The other part, the visceral skeleton, consists of a series of 

 bars (visceral arches), an arch between each two visceral clefts 

 (including the mouth) and an arch behind the last cleft. In the 

 higher vertebrates the distinction between these two parts of the 

 skull is hardly -evident at the first glance. 



Both cranium and visceral skeleton are outlined in cartilage, and 

 in Cyclostomes and Elasmobranchs never pass this stage. In all 

 higher classes more or less of the cartilage is transformed into bone, 

 and these cartilage bones are supplemented by other bones of mem- 

 branous origin, this holding especially for the cranial roof and the 

 jaws. As a rule, the higher in the scale, the greater the extent of 

 ossification, but the number of separate bones is greater in most 

 lower groups than in the higher, the result, in part, of fusion, in part 

 of complete loss of bones which occur in the lower forms. 



Formerly the skull was regarded as a complex of fused and differentiated 

 vertebrae (Vertebral theory of Oken and Owen), the vertebrae being modified 

 by the great size of the brain, by the inclusion of the organs of special sense, 

 and by the development of the anterior part of the alimentary canal. In its 

 broader features this idea was overthrown half a century ago, but that vertebrae 

 are included in the cranium is beyond doubt, for it is distinctly composed of an 

 anterior non-vertebral part, and a posterior part in which several vertebrae 

 are easily seen in the embryo (fig. 65). No sclerotomes are recognized in front 

 of the ear, while behind it their individuality is soon lost, the resulting mesen- 

 chyme forming a continuous layer between the brain on one side and the myo- 

 tomes and epidermis on the other. From this continuum enclosing the whole 

 brain and the sense organs, the cranial structures are developed. 



The visceral arches do show a marked metamerism, as do the visceral clefts 

 between which they lie, but whether this metamerism corresponds to that of the 

 trunk or is a special 'branchiomerism' has not been settled, although the former 

 seems more probable. Another question is whether the whole visceral part 

 belongs to the head. On the one hand is Amphioxus where the very numerous 

 clefts extend from near the tip of the head through nearly half of the body. 

 In Cyclostomes the clefts may number fourteen, all being supphed by branches 

 of the vagus (tenth cranial) nerve, the last lying some distance behind the brain. 

 The number of clefts and arches diminishes progressively in ascending the 



1 Recentlj' the terms neurocranium and viscerocranimn have been introduced for 

 these. Strictly speaking the cranium is neural, and thus the term is used here. 



