SKULL 871 
(p. 112), and those belonging to visceral arches which give rise to the 
Hyorp 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. Roughly 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 cnogenetic 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 from 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) Prasphenoid ; (4) 
continuation of the last into the interorbital and internasal Septum. 
IL. 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;1 (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 
1 Jn several species of CoRMORANT a peculiar, long, pyramidal sesamoid bone 
is loosely attached to the supra-occipital, serving apparently to increase the 
surface of attachment of the musc. complexus capitis and of part of the temporal 
muscle, 
