294 PROF. "W. K. PAEKER ON THE STEUCTUEE AND 



sphenoid (b.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 (fJ.s) ; behind, it 

 has a gently concave form, and is separated from the convex fore margin of the basi- 

 occipital (b.o). 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 (PI. 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.cl) ; 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 (i-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 i\ie 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 (PI. LXX. fig. 3, l.eu, b.o). 



A transverse section through the hind skull, exposing the tympanic cavity {c.ty), with 

 its traversing columella (PI. 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 (j), externally, and the basis cranii within. 



In the next section (PI. LXXI. fig. 6) the alisphenoid [al.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 (PI. LXXI. fig. 5, al.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 (PI. LXIX. fig. 7, and PI. 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 {i.tr), which is covered with a plaster of cartilage formed by the 



