190 Charles Searing Mead. 



the forerunner of the later well-developed osseous roof, the tegmen 

 tympani, a structure which occurs for the first time in the mam- 

 mals. 



Comparing the hiatus in Sus and in mammals generally with 

 the foramen nervus facialis in the Sauropsida and Amphibia, we 

 are struck with a great difference in two important points; in the 

 first place, the hiatus in the mammals lies in the ear-capsule; and 

 secondly, it is situated within the cranial cavity in the adult, so that 

 the nervus petrosus runs for a short distance through the cranial 

 cavity after it leaves the hiatus. Gaupp explains these two points 

 in connection with the changes which the mammalian cranial cavity 

 has undergone, as follows (1900, p. 509) : "The intercapsular posi- 

 tion of the foramen facialis is a result of the growth of the cochlea. 

 In the amphibians it lies in the boundary between the ear-capsule 

 and the solid basal plate; in the lizards the most anterior part of the 

 capsule which contains the cochlea is ventral to this foramen; in 

 the mammals we find this foramen on the dorsal edge of the ear- 

 capsule ; a portion of the capsule is not only ventral to, but is also in 

 front of, the foramen. These changes are comprehensible, when one 

 recognizes that in the mammals the ductus cochlearis has grown into 

 that part of the skull which, in the lizards, forms the lateral part 

 of the anterior half of the basal plate. One also notices a reduc- 

 tion in the size of the pars vestibularis. The bridge, therefore, which 

 roofs over the first portion of the facial canal in the mammals, has 

 its homolog in the lower vertebrates in the prefacial commissure. 

 That one cannot speak of this foramen in the mammals as being 

 basicapsular but intercapsular, is due to the fact that that part of 

 the cranium which forms the cochlear part of the capsule in the 

 mammals is, in the reptiles, an undivided part of the basal plate," 



Let us come back now to the facial nerve. As stated above, im- 

 mediately in front of the primary portion of the facial canal it gives 

 off a branch, the nervus petrosus superficial is major (ramus pala- 

 tinus) which passes out of the hiatus canalis facialis. The petrosus 

 passes through the ganglion geniculi, which also covers a part of the 

 main trunk of the facial nerve, then goes forward and downward, 

 passing close under the processus alaris (Fig. Y, n. p. s. m.) and 



