

SKULL AND VISCERAL ARCHES IN LEPIDOSIREN AND PROTOPTERUS. 55 



the posterior wall of the Gasserian recess, and completes the outer boundary of the 

 hyoideo-mandibular foramen. Internal to this the trabecula is perforated by a small 

 foramen for the superior palatine branch of the seventh nerve (fig. 8, vii. sup. pal.). 

 In front of the Gasserian recess the trabeculse are raised into the orbito-temporal pro- 

 cesses. From the outer side of this is growing back a curved cartilaginous lamina 

 forming the outer wall of the front end of the Gasserian recess. In consequence of the 

 mode of formation of this wall, the ophthalmicus profundus of v. perforates it in an 

 antero-posterior direction. The upper rim of this wall (twnia marginalis) is growing 

 back to fuse with a forward growing process of the posterior wall of the Gasserian 

 recess. By their junction the foramen pro-oticum (temporal foramen) is enclosed, 

 transmitting the superior and inferior maxillary branches of v., the buccal and super- 

 ficial ophthalmic branches of vii., and also the ramus communicans between the lateral 

 line systems of vii. and x. (v. 2 , v. 3 , vii. lat.) ; cf. figs. 14 and 16. 



The orbito-temporal process extends a little further forward than in stage 31, and 

 encloses the foramen for the oculo-motor. This nerve emerges on the outside in the 

 groove formed by v. 1 after emergence from the Gasserian recess, and joins the latter 

 nerve. Owing to the elongation of the front part of the head, the optic nerve no longer 

 passes out close in front of the orbito-temporal process, but has been pulled forwards 

 (fig. 8). The trabeculse are continued much further forward than in stage 31. They 

 are produced ventral to the optic nerve, at first as vertical laminae, further forward 

 becoming horizontal, and pushed upwards by the palatine symphysis. Above this they 

 fuse into a median internasal septum. Behind this the trabeculse are connected by a 

 sheet of connective tissue which chondrifies later to form the " cartilaginous basis 

 cranii" of Bridge (fig. 9). 



The basicranial fontanelle enclosed by the trabeculse is covered in as to its posterior 

 half by the parasphenoid, which is in this region fused into a median plate. A vacuity 

 is, however, still left underneath the hypophysis. The original paired nature of the 

 parasphenoid is also seen in the two long processes which project backwards under the 

 occipital region. The front end of the fontanelle is open. 



The internal carotids enter the skull between the inner edges of the trabeculse and 

 the parasphenoid, a position which they retain throughout life. 



Fig. 14 shows that the palato-pterygoid bone has completed its backward growth, 

 and overlaps the inner surface of the quadrate. Owing to the elongation of the anterior 

 region of the head, this slopes more forward than in stage 31. It is now attached by 

 two processes, the processus oticus and the original one, the processus basalis. It is 

 impossible to speak of a processus ascendens as distinct from the basalis. 



The splenial is still growing back along the inner surface of Meckel's cartilage, but 

 has not yet begun to curve over on to its outer side. 



The sole representative of the opercular apparatus is a free splint of bone 

 {operculum) in the position shown. 



In fig. 15 is shown a reconstruction from sagittal sections of Protopterus, about 



