THE SKULL OF ACANTHIAS VULGARIS 429 



openings, while sections of the labyrinth region show that the 

 central opening is that of the eighth nerve. A dissection of the 

 cranium in this region was also made, following along the nerve 

 canals. 



Ventral to these foramina and below their common recess is 

 a small opening for the sixth (abducens) nerve (VI). This 

 opening lies between the trigemino-facialis foramen and the 

 foramen of the eighth nerve. Behind the foramen for the main 

 trunk of the glossopharnygeal is a small opening for a second 

 branch of the same nerve (IX) ; and posterior to this is the large 

 foramen for the main trunk of the vagus nerve (X), with a 

 small opening for a minute branch just behind it. 



Above the cut edge of the cranial roof in this region (that of 

 the fossa endolymphaticus) is, posteriorly, the perilymph duct 

 (per.d.) and anterior to it — the opening of the endolymph 

 duct (e.d.). 



The labyrinth region was studied by means of transverse 

 sections through the otic capsule of the left side of an adult 

 cranium (figures 11 to 22 inclusive). These were free hand 

 sections about 2 mm. in thickness. In each case the anterior 

 side of the cut was drawn, twice the natural size. Only the 

 skeletal labyrinth is represented, no attempt being made to 

 show the membranous parts. The primitive condition of the 

 membranous labyrinth of Acanthias is evident, however, from 

 the corresponding simplicity of the skeletal labyrinth. This is 

 well shown in sections 16, 17 and 18. The vesicle is practically a 

 single chamber, with only suggestions of a constriction into 

 saccular and utricular regions. The connections of superior, 

 horizontal and posterior ampullae with the otic chamber are 

 seen in sections 13, 14 and 19 respectively. In sections 15 to 18 

 inclusive, the dorsal part of the auditory vesicle is occupied by 

 the perilymph cavity, the divisons being membranous and so not 

 shown in the drawings. 



The visceral skeleton consists of a series of seven cartilagi- 

 nous arches so arranged as to form the skeletal elements of 

 the mouth and the support of the gill clefts (figs. 5 and 6). The 



