Tho Cbondrocriiuiiiiu of an Embryo Pig. 189 



and serves for the passage of the ncrvus acusticus (PL I). This 

 opening is roughly dumh-bell-shaped, with its axis placed in a plane 

 lying almost vertical. Its lower portion serves for the passage of 

 the ramus cochlearis, nerviis acusticus, while the ramus vestibularis 

 passes through the upper portion. These nerves are distributed over 

 the inner part of the ear-capsule and need not concern us further. 



The anterior opening in the meatus acusticus internus is for the 

 passage of the nervus facialis. Anteriorly it is bounded by a slender 

 rod of cartilage (r. for. fac), which extends from the anterior part 

 of the pars vestibularis to the dorsal part of the ])ars cochlearis. 

 This short stretch, the only place where the facial nerve is completely 

 surrounded by cartilage, represents the canalis facialis and its open- 

 ing is the hiatus canalis facialis of Vrolik. He believed (Vrolik, 

 1873) that the hiatus was the homolog of the external opening of 

 the foramen facialis in the lower vertebrates. Directly in front of 

 the foramen is the ganglion geniculi, which is continued forward by 

 the nenais petrosus superficialis major; the main stem of the facial 

 nerve curves outward and Ijackward onto the outer side of the 

 auditory capsule. 



In Talpa, Fischer (1901 b) found a double bridge from the 

 vestibular to the cochlear part of the auditory capsule, or, as he 

 expressed it, a broad connection which is perforated by an opening 

 through which passes the nervus petrosus superficialis major. He 

 found the same condition also in one of Hertwig's models of the 

 human ear-capsule. He rightly identified this opening as the hiatus 

 canalis facialis, but from the evidence quoted he concluded that tiiis 

 condition is constant in the mammals (p. 504) and that the hiatu^ 

 is only a perforation of the cartilaginous wall of the primitive facial 

 canal. Van Kampen (1905, p. 387) supports him in this belief. 

 I am inclined to uphold Vrolik's opinion and, therefore, look upon 

 the cartilaginous bridge over the facial canal distal to the hiatus as 

 being altogether secondary, a fact supported by its total absence in 

 the crania of the Sauropsida and Amphibia and by its usual absence in 

 mammalian chondrocrania. Where such a secondary bridge occurs in 

 the embryo, we always find it well developed in the osseous skull of 

 the adult. Hence, I believe that this cartilaginous bridge is merely 



