1432 



HUMAN ANATOMY. 



Fig. 



necessary room for expansion, its growth is retarded. After the sixth year, when 

 the eruption of the permanent teeth begins, the antrum loses its general spherical 

 outline and gradually acquires the definite pyramidal form. 



The frontal sinus formed as an extension of the nasal fossa during the third 

 foetal month, is for a time so small that it is usually regarded as absent at birth. 

 Although indistinctly seen during the third year, not until about the seventh is the 



'sinus a definite space ; it remains small, how- 

 ever, until puberty, after which its adult 

 proportions are gained. 



The sphenoidal sinus, primarily arises 

 by the constriction and partial isolation of a 

 part of the primitive nasal fossa. Although 

 its development begins during the third 

 foetal month, the space remains so rudimen- 

 tary that not until the seventh year has 

 absorption of the cancellous bone progressed 

 sufificiently to make the sinus apparent. 



Notwithstanding its rudimentary condi- 

 tion in man, the organ of Jacobson devel- 

 ops at a very early period, beginning as a 

 groove-like depression on the median wall of the nasal pit. This groove is converted 

 into a tubular pouch that soon becomes laterally compressed and, by the middle of 

 the third month, measures about .5 mm. in length and receives twigs from the olfac- 

 tory nerve (Kallius). After the fifth foetal month the organ suffers regression and 

 becomes rudimentary and variable in comparison with the perfection it attains in 

 animals possessing olfactory sense in a high degree. 



The development of the outer nose is closely associated with the changes 

 affecting the median and lateral nasal processes — prominences considered in connection 

 with the formation of the upper boundary of the primitive oral cleft (page 62). 



Reference to Fig. 1 192 shows the median nasal processus, separated by a distinct 

 furrow that soon becomes filled and partially obliterated by ingrowth of young 

 connective tissue, as does likewise 



1191. 



Fore-brain 



Infranasal area 



Nasal fossa 



Lateral nasal process 

 Median nasal process 



Processus globularis 

 Maxillary process 



Mandibular process 



Anterior end of head of foetus 10.5 mm. in length, 

 showing early development of external nose. X S. 

 (Peter.) 



Fig. 1 192. 



/ 



Fore-brain 



the groove between the globular and 

 maxillary processes. At first sepa- 

 rated by a relatively wide interval, 

 the infranasal nasal area of His, the 

 nasal apertures are brought nearer 

 together by the rapid narrowing of 

 the interposed portion of the frontal 

 process. Eventually the tissue be- 

 tween the globular processes becomes 

 the philtrum of the upper lip and 

 that between the nasal openings 

 persists as the partition between the 

 nostrils. By the end of the second 

 month the external nose is defined, 

 but is very broad and flat and lim- 

 ited above by an arched furrow that 

 separates the convex nasal margin 

 (His) from the forehead. The nos- 

 trils, originally placed high and for 

 a long time directed forward, grad- 

 ually descend and assume a hori- 

 zontal plane as the middle of the arched nasal margin grows downward and forward 

 to become the point of the nose. These changes, however, are not accomplished 

 until near the end of gestation and at birth the bridge of the nose is still small and 

 flat which, in connection with the general breadth of the organ, imparts to the 

 infantile nose its peculiar stumpy appearance. Not until long after birth, and, indeed, 

 not until after puberty, does the outer nose acquire its definite individual form in 



r /-*> 



nfranasal area 



^ Nasal aperture 

 ^>|^Lateral nasal process 

 fjJT^ Medial nasal process 

 .^ — Processus globularis 



'""^ Mandibular process 



\ 



\ 



Head of fcetus of about 20 days, showing developing nose 

 X 13. (Rabl.) 



