406 CHARLES CLIFFORD MACKLIN 



is almost straight, in the sagittal plane, there being a slight 

 depression between the superior and the supraconchal (Sakter- 

 wulst — Voit) prominences. In the coronal plane the tectum is 

 convex upwards, and, when regarded from above, it is seen to 

 be widened by the afore-mentioned prominences. 



The paries nasi may be divided into a smooth dorsal portion, 

 the planum antorbitale (fig. 3), which forms the ventro-medial 

 wall of the orbit, and a ventral portion, which presents a much 

 more uneven surface, the two grading into one another in the 

 region of the lacrimal duct. Mead states that ''in the reptiles 

 a line joining the corresponding place (commissura sphenoeth- 

 moidalis) with the processus maxillaris posterior would separate 

 the paries nasi from the planum antorbitale. The same is true 

 also of the mammals, although here the planum antorbitale is 

 usually oblique instead of transverse." Upon comparison with 

 such forms as the pig the paries nasi (sensu stricto) of man, com- 

 prising the part ventral to this line, is very rudimentary. Dor- 

 sally the planum antorbitale terminates in a rather sharp point, 

 and upon examining the inner surface of the ectethmoid it is 

 seen that this tip is turned inward and forward, the cartilage be- 

 ing directly continuous, to form the processus cupularis posterior 

 (figs. 12 and 14). The border above this extends upward to 

 the bridge of cartilage joining the dorsal portion of the tectum 

 with the septum; it presents near its cranial extremity a small 

 notch (fig. 12), and below this it is fitted closely to the contour 

 of the septum within, the narrow space between being known as 

 the cupulo-septal fissure (figs. 2 and 14) completely filled with 

 connective tissue. This fissure marks off, upon the septum, the 

 delimitation between its interorbital and nasal portions. The 

 upper portion of the planum antorbitale is in close relationship 

 to the ventral root of the ala orbitalis on account of the very 

 rudimentary condition of the interorbital septum in the mammals. 

 A theory accounting for the shortening of the latter is given by 

 Mead who states: "In the evolution into the nasal capsule of 

 the mammals the posterior part of the capsule of the reptiles 

 has been expanded by the backward rotation of the posterior 

 wall (reptilian planum antorbitale), the pivot being the more 

 solid lateral side." 



