SKULL OF A 43-MM. HUMAN FETUS. 



83 



The middle concha is well marked and widest in 

 its central region. It overhangs the middle 

 meatus and does not show a perforation, as it 

 did in la. Posteriorly it does not pass over upon 

 such a distinct ridge joining the posterior border 

 of the inferior concha as in la, but there is a low 

 ridge of young cartilage joining the two and 

 development is evidently proceeding rapidly here. 



The middle meatus is very capacious. Its floor, 

 as already observed, is steeper in front than behind 

 and it continues forward into the cavity formed 

 by the middle prominence; beyond this the con- 

 cavity leads downward into the anterior naris. 

 The deepest part of the meatus is the posterior. 

 About its middle is the process of the middle mea- 

 tus, altogether of precartilage, which represents 

 the isolated cartilage of the middle meatus of la, 

 together with its pedicle of precartilage. It pro- 

 jects almost straight backward for 600 micra to 

 end freely. It may represent the uncinate pro- 

 cess. The connection with the wall is at the 

 medial side of the posterior prominence. The 

 cartilage of the wall is quite thick here and the 

 part joined to this process is partially separated 

 from that lying laterally by an upwardly-directed 

 cleft. 



The inferior concha needs no further description. 



The inferior meatus, below the inferior concha, 

 is seen in figure 41. The floor, in front, is formed 

 by the paraseptal process, while behind this it is 

 made by the developing premaxilla and maxilla. 

 The paraseptal process comes almost into contact 

 with the cranial end of the condensed mesenchyme 

 of the premaxilla and the post-transverse incisure 

 above it leads backward into the meatus. The 

 end of the nasolacrimal duct is shown in figure 41. 

 The inferior meatus is very narrow as compared 

 with the middle. 



Just in front of the paraseptal process is the 

 concavity formed by the inferior prominence. 

 This is quite evidently the wall of the future 

 vestibule. Above the indentation for the middle 

 prominence is a low ridge which probably repre- 

 sents the agger nasi. It is continued backward 

 to the anterior end of the middle concha. 



The mucous membrane of the outer nasal wall is 

 shown in figure 38, which should be compared 

 with figure 41 drawn from the same point of view. 

 We note all the turbinate folds, covering their 

 respective cartilaginous conchas. The superior 

 membranous concha is the shortest and least dis- 

 tinct and the two lower are very well marked. 

 The superior meatus is short and shallow and 

 the middle one is deeper and forms a very dis- 

 tinct cleft. The inferior meatus, longest of all, 



is quite shallow but projects into the meatus as a 

 wide double layer of epithelial cells. This is 

 shown from without in figure 39, where it is 

 seen that the epithelium underlies practically the 

 inner half of the inferior concha. In figure 41 the 

 tip of the nasolacrimal duct, which is here some- 

 what expanded, is seen projecting into the space 

 of the inferior meatus; its relation to the epithe- 

 lium is seen in figure 39. 



The caudal extremities of the meatuses show 

 the familiar approximation where the nasal cap- 

 sule narrows into the elongated nasopharyngeal 

 canal, as shown in figure 38. This canal is sup- 

 ported laterally by the palate and internal ptery- 

 goid bones, in their common investment of 

 mesenchyme (fig. 41). 



NASOLACRIMAL DUCT SYSTEM. 



The nasolacrimal duct system was modeled 

 and a view of it is seen in figure 39. Commencing 

 laterally, we note the two lacrimal ducts, ter- 

 minating in the punctae lacrimalis and opening 

 into the somewhat dilated nasolacrimal sac. 

 From this point the nasolacrimal duct runs 

 directly backward for some distance, crossing 

 above the maxilla and coming to lie between the 

 lacrimal bone and the tip of the paranasal process. 

 The duct does not lie in the depths of the notch 

 which is made by this process and the posterior 

 maxillary process. It now curves downward, 

 inward, and backward, under the posterior 

 maxillary process, terminating in an expansion 

 (fig. 40) which is contiguous with the epithelial 

 plate of the future inferior meatus (fig. 39). 

 It has not yet broken through the epithelium. 

 Throughout its extent no definite lumen can be 

 recognized in this system, though the epithelial 

 cells are arranged in ring-like formation with a 

 potential lumen within. 



The lower portion of the nasolacrimal duct in 

 the human fetal skull is situated some distance 

 posterior to that in the lower forms, as (to cite one 

 of numerous examples) the chondrocranium of the 

 cat (Terry), where the duct passes forward lateral 

 to the anterior transverse lamina to empty into 

 the nasal cavity in front of this lamina. The 

 anterior transverse lamina is represented, in the 

 human chondrocranium, by the paraseptal proc- 

 ess and, if the position of this part of the duct 

 were the same in homo as in the lower forms, it 

 would pass forward around the paraseptal process 

 to connect with the mucous membrane of the nose 

 in front of this process. It would then empty into 

 an extension of the anterior naris, as it does in the 

 lower forms. Instead, it is situated far behind the 



