Mammalia: Nervous System 



689 



Fig. 514. Lateral surfaces of models of the membranous portion of the left 

 internal ear from human embryos. (Different enlargements.) From embryos of 

 (A) 6.9 mm., (B) 10.2 mm., (C) 13.5 mm., and (D) 22 mm. (am) Ampulla; (c.v.) 

 cecum vestibulare of (d.c.) cochlear duct; (d.e.) endolymphatic duct; (d.s.L, d.s.p., 

 d.s.s.) lateral, posterior, and superior semicircular canals; (sac) sacculus; (ut. ) 

 utriculus. (After His, Jr. Courtesy, Bremer: ''Text-Book of Histology," Phila- 

 delphia, The Blakiston Company.) 



The mammalian utriculus, with its three semicircular canals, 

 differs in no important way from that of other vertebrates. In the 

 saccular region the conspicuous mammalian feature is the greatly 

 elongated lagenar outgrowth (Fig. 180). It appears as a tube coiled 

 into a close spiral. The form of the bony case enclosing it suggests a 

 snail shell; hence the name, cochlea. The number of turns in the 

 cochlea varies with the length of the tube, ranging from less than one 

 full turn in monotremes to as many as five in a South American rodent, 

 Coelogenys. Insectivores and cetaceans usually have about one and a 

 half turns; ungulates and carnivores, three to four; man, between two 

 and a half and three. The cochlea is a small structure compared to the 

 size of the animal. The human cochlea measures some 25 to 30 mm. in 

 its spirally coiled length. The junction of the cochlea with the sacculus 

 is somewhat sharply constricted. 



The wall of all parts of the definitive otic sac, except the relatively 

 small regions occupied by sensory cells, is a very thin membrane 

 consisting of the internal ectodermal epithelium, which is only one cell 

 in thickness, and, externally, a delicate layer of connective tissue. The 

 complete sac, with its appended canals and cochlea, is of such com- 

 plexity of form as to merit the name membranous labyrinth. 



The fully developed membranous cochlear tube is not circular in 

 cross section but more nearly triangular (Figs. 515, 516). Its wall is 

 very thin except on one of the three flattened sides, where it becomes 

 much thickened and possesses an elaborate histologic differentiation 

 which constitutes the essential nervous receptive mechanism of hearing 



