!518 . EMBRYOLOGY. 



capsule they receive in Man as early as the second month a support, 

 which subsequently ossifies. In many Mammals the turbinated 

 processes acquire a complicated form owing to the production upon 

 the first fold of numerous smaller secondary and tertiary folds, which 

 become peculiarly bent and rolled up. On account of the complicated 

 form resulting from the production of the turbinated processes the 

 olfactory sac has received the name of olfactory labyrinth. 



Thirdly and lastly, the mucous membrane of the nose is increased 

 in extent by the formation of evaginations which grow out partly 

 into the ethmoid region of the cranial capsule, which consists of 

 cartilage during early stages of development, and partly into a number 

 of the covering bones (Belegknochen). 



In this manner are formed the numerous small cribriform pits in 

 the cartilaginous cribriform plate. Somewhat later (in Man during 

 the sixth month) an evagination into the upper jaw is developed into 

 the antrum of Highmore. Finally, after birth evaginations penetrate 

 into the body of the sphenoid bone and into the frontal bone, pro- 

 ducing the sinus sphenoidales and sinus frontales, which, however, 

 attain their full development only at the time of sexual maturity. 

 In many Mammals the enlargement of the nasal cavity takes 

 place even farther backward into the body of the occipital bone 

 {sinus occipitales). Inasmuch as the accessory cavities of the nose 

 take the place of bone-substance, they natiu^ally contribute to the 

 diminution of the weight of the cranial skeleton. 



In connection with the account of the organ of smell the formation 

 of the external nose ought now to be briefly considered. It is 

 developed out of the frontal process and the parts designated as 

 nasal processes (figs. 286, 288, and 289), these becoming elevated more 

 and more above the level of the surrounding parts. At first broad 

 and bulky, the nose later becomes thinner and longer and acquires 

 characteristic forms. The nostrils, which at their formation are far 

 apart, come together in the median plane. Whereas the distance 

 in an embryo five weeks old is, as His has shown by measurements, 

 1*7 mm., it has become reduced in an embryo seven weeks old to 

 1'2 mm., and in one somewhat older to 0'8 mm. The median frontal 

 process is correspondingly reduced in thickness and furnishes the 



nasal septum. 



Summary. 



1. The organ of smell is developed out of two pit-like depressions 

 of the outer germ-layer, which are formed on the frontal process at 

 a considerable distance from each other. 



