PRIMORDIAL CRANIUM OF THE CAT 335 



olfactory fenestra extends in the form of three bays or sinuses. 

 In the present stage there is no cartilaginous cribriform plate; 

 its place is occupied by mesenchyma perforated by the olfactory 

 nerves. These are in two groups, one near the septum, the 

 second represented by bundles passing through the sinuses at the 

 side of the window. The sheet of mesenchyma perforated by 

 the olfactory nerves does not quite fill the fenestra olfactoria. 

 It is closely applied to the septum nasi, then crosses the mid- 

 line to be continuous with the membrane of the opposite side; 

 it is fixed at the anterior boundary of the window (tectum nasi) . 

 Posteriorly the membrane passes over the lamina infracribrosa 

 against which it is closely applied. In the lateral direction it 

 becomes continuous with the membrane filling the spheno- 

 ethmoidal fissure. This is fixed to the dorsal but not to the 

 ventral boundary of the fenestra, which, it will be recalled, is 

 formed by the antorbital plate. In relation to this membrane 

 is the lateral branch of the nasal nerve (fig. 26). The latter 

 appears in the sections through the ajiterior part of the orbit, 

 running toward the spheno-ethmoidal fissure. This it enters, 

 passing ventrad of the membrane, a position it retains in its 

 course forward to the olfactory fenestra. Here it enters the 

 nasal capsule, pursues a short course through the crista semi- 

 circularis and then passes to the exterior by one of the epiphanial 

 foramina. 



Septum nasi (figs. 1, 2, 25 to 30). This median partition is 

 the direct continuation of the trabecular plate forward into the 

 ethmoidal region. It is low and broad cau dally where it lies 

 between, and enters into the median walls of the posterior 

 cupolae. As it extends forward the dorsal margin ascends, the 

 height of the septum increasing gradually toward the fenestrae 

 olfactoriae; between the olfactory fenestrae it increases rapidly 

 and reaches its maximum height at the tectum nasi; then de- 

 creases gradually in approaching the level of the fenestrae 

 narinae. The ventral margin of the septum presents a longitu- 

 dinal concavity, and is thickened between the paraseptal car- 

 tilages and again opposite the vomer. Anterior to the para- 

 septais it is united on each side to the anterior transverse lam- 



