294 PROF. W. K. PARKER ON THE STRUCTURE AND 
sphenoid (J.s) is composed of a wide square hind part, and of a narrow fore part, which 
has the outline of a truncated cone; it is, however, flat above and rounded below. ‘The 
ossification in front stops suddenly against the presphenoidal cartilage (p.s); behind, it 
has a gently concave form, and is separated from the convex fore margin of the basi- 
occipital (6.0). Beneath the narrow synchondrosis is the middle Eustachian opening 
(m.eu), and on each side of that the bony matter projects as two small ears inside the 
larger ears formed by the basitemporals. The basisphenoid also gives off a pair of 
lateral ears before it narrows in close to the middle; at that part there are two smallish 
foramina looking forwards. The middle line, below, of the hind part is grooved, 
the sides are convex. Above (Pl. LXX. fig. 4) the hollow and shelving upper table is 
square, with the corners trimmed off; the sides next to the cartilage are straight; both 
the hinder and the free front edge are gently cut away in an arcuate manner. The 
fore edge is tilted upwards; it is the postclinoid wall (p.c/); behind its shelving face 
the floor is gently hollowed for the medulla oblongata, but it has a slight median ridge 
behind. The pituitary cup is oblique, and looks forwards and upwards; it has two 
large openings below and behind for the emergence of the internal carotids (¢.c). The 
body of the bone behind the pituitary cup is one large air-cell, which opens antero- 
laterally into the air-cavity of each alisphenoid into the common middle opening (m.eu). 
That part of the labyrinth of pneumatic passages which is common to the outside of the 
prootic and the inside of the quadrate also opens into the side of the basisphenoid at 
its middle. Thus the cavity of the bone has five communications with the labyrinth, 
right and left and behind. The specialization of the first pair of clefts is so great as to 
throw them both into one complex system of passages, pervading the whole hind skull, 
auditory capsules included, and including also, as I shall soon show, the hinder or 
articular part of the mandibular rods. The internal carotids enter the skull in front of 
the pouches where the lateral Eustachian tubes communicate with the pneumatic 
openings of the basioccipital (Pl. LXX. fig. 3, l.ew, 6.0). 
A transverse section through the hind skull, exposing the tympanic cavity (¢.ty), with 
its traversing columella (Pl. LXXI. fig. 7, co), shows the relation of the basisphenoid 
to the fore part of the auditory labyrinth (a.s.c, vb), and the manner in which the 
pneumatic passages enter the quadrate (q), externally, and the basis cranii within. 
In the next section (Pl. LX XI. fig. 6) the alisphenoid (a/.s) is shown as cut down the 
middle immediately above the fore part of the pituitary body (py) and the infundi- 
bulum (inf’). 
But the widest part of the cranial cavity is across the thin bulging part of the ali- 
sphenoids (Pl. LXXI. fig. 5, a/.s); at this part they are ossified to their top edge, but 
below have a soft tract which ends a little above the presphenoid (p.s). Passing still 
further forwards (Pl. LXIX. fig. 7, and Pl. LXXI. fig. 4) we see how the three cartilages 
have built the large orbito-nasal dividing wall, the main mass of which has been formed 
by the intertrabecula (¢.tr), which is covered with a plaster of cartilage formed by the 
