SKULL 871 



(p. 1 1 2), and those belonging to visceral arches which give rise to the 

 Hyoid apparatus, the palate and the jaws (Maxilla, Mandible). 

 Being a most complicated structure, the composition of the Skull 

 will be better understood if the description of its adult condition be 

 prefaced by a short account of its general development. The first 

 trace of the Cranium appears towards the end of the fourth day of 

 incubation (Embryology, p. 209), and on the seventh (p. 211) its 

 essential shape is completed. Eoughly speaking, the ventral half of 

 the cranial capsule consists of a mass of cartilage in which no separate 

 elements are distinguishable, while its side-walls and roof, though 

 continuous with that mass, are still in a membranaceous condition, 

 being formed of indifferent connective tissue. The inmost layer of 

 this membranaceous covering remains throughout life as the dura 

 mater (Nervous System, p. 622), while the outer and thicker layer 

 ossifies, of course to form membrane BONE (p. 47), and the 

 greater part of the cartilaginous framework is likewise converted 

 into bone. Though so unlike in their origin, it is not always easy 

 to distinguish between these two sorts of bone, owing to the con- 

 densed or abbreviated way in which stages of development are 

 hurriedly passed through and to other ca^nogenetic changes 

 (Anatomy, p. 14) which obscure and sometimes completely alter 

 the proper phylogenetic procedure. Thus bones originally cartila- 

 ginous are overlaid by direct ossification of membrane, and often 

 have their cartilage more or less suppressed, so that they appear 

 from the beginning as formed from membrane and not from 

 cartilage. This applies to most of the jaw-bones, as well as those 

 of the roof of the mouth or palate. Most of the bones of the Skull 

 ossify each fi'om one centre, and originally all are paired. 



Analysis of the Cranium shews it to be constituted thus : — 



I. The basis of the cranial capsule, composed of the following 

 (cartilaginous) elements, which, proceeding from behind forwards, 

 are — (1) Basioccipital ; (2) Basisphenoid ; (3) Prsesphenoid ; (4) 

 continuation of the last into the interorbital and internasal Septum. 



II. The right and left sides, formed by the (cartilaginous) paired 

 — (1) Lateral Occipitals (Exoccipitals of many authors); (2) Peri- 

 otics ; (3) Alisphenoids ; (4) Orbitosphenoids ; (5) Ethmoids. 



III. The roof of the capsule, formed by the following (membrane) 

 bones in pairs — (1) Supra-occipitals ; ^ (2 and 3) Parietals and 

 Squamosals; (4) Frontals ; (5) Lacrymals ; (6) Nasals. 



IV. Additional (membrane) bones on the ventral side of the 

 base of the capsule — (1) a pair of Basitemporals, amalgamated 

 with the Basisphenoids ; (2) an unpaired investment of the likewise 



^ In several species of Cormorant a peculiar, long, pyramidal sesamoid bone 

 is loosely attached to the supra-occipital, serving apparently to increase the 

 siu'face of attachment of the muse, complexus capitis and of part of the temporal 

 muscle. 



