594 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
plates, is made imperfect by the implantation of these capsules, the arches that should 
grow ventrad, like the costal arches, miss their point of attachment, and their nervous 
supply has to find its way round the obstructive masses. 
The middle pair of capsules are free ; they do not blend themselves with the chon- 
drocranium, but they affect its growth by causing it to harmonize with their size, form, 
and mobility. 
In flat, broad skulls this is the less seen ; but in high, compressed skulls the two 
parallel bands suddenly unite in front of the pituitary body, and grow up into a high 
wall between the two capsules that mutually become approximated. 
In these cases the brain-cavity is lifted up high above this partition, and any walls it 
may acquire grow as wings from the top of this interorbital crest. 
Further forwards we have a similar state of things, namely, in the nasal region, these 
foremost paraneural capsules often approximating very closely. 
Here the crest is continuous with that between the eyeballs ; but the roof of the 
nasal labyrinth, right and left, coalesces with the ascending trabecular crest. The cap- 
sules here again are blended with the axis. 
Only in the lower (ichthyic) types do the basal plates grow up over the brain mass, 
except behind ; so that we have the chondro cranium as a basin or a trough, and not as a 
finished and roofed structure. This is a great specialization of a neural-arch structure. 
The closing in in front of the neural cavity is not necessarily directly over the end of 
the skeletal axis, and the nasal septum is continued in front of the cranial cavity. 
The ventral arches of the head are for several reasons very much unlike costal 
arches. They are only perfect behind the mouth, and very independent in their 
growth. 
Costal arches spring, normally, from a vertebral centrum, as directly, indeed, as neural 
arches do ; but the cephalic arches may or may not be attached, even secondarily, to the 
bands that answer to the distinct vertebral centra. 
Where the costal arches grow, there the descending or ventral laminae are divided 
into a “ splanchnopleure ” and a “ somatopleure,” the pleuro-peritoneal space being 
between the two, and these arches are formed in the outer layer. 
But this division, because of the closing in of that space, does not exist below the 
head ; the old term “ visceral arches ” still may be used for the bars. They are, to all 
intents and purposes, jpleural arches, and they lie close, or near, the outer wall of the 
throat. 
When the visceral arches are attached to the skull-base it is often a secondary attach- 
ment, and they, most of them, have a habit of growing over the top of the pharynx, 
leneath the axis. 
The intervening clefts, the mode of subdivision of the bars, the structures that are 
attached to them, and the bony plates that round the mouth are applied to them as 
investing bones ; in all these things the visceral arches are diverse from the costal . 
