75] THE SKULL OF AM I URUS— KINDRED 75 



nally by the suprasphenoid. The former is usually sutured to the alisphenoid 

 between the ventro-posterior margin of the optic foramen and the antero-ventro- 

 margin of the trigemino-facial foramen. An anterior process of the alisphenoid 

 extends ventrally as the anterior margin of the optic foramen, descending in the 

 orbital wall as far as the orbitosphenoid. This interdigitation between the ali- 

 and orbitosphenoids continues dorso-posteriorly to the anterior end of that part 

 of the frontal which overlaps the dorsal margin of the ahsphenoid. The latter 

 extends higher up in the cranial wall than does the more anteriorly situated 

 orbitosphenoid. Above the trigemino-facial foramen the alisphenoid interdigi- 

 tates with the anterior margin of the ventral part of the sphenotic, the line of 

 interdigitation continuing forward between the sphenotic and the frontal. 

 The sphenotic projects broadly above this part of the alisphenoid and a con- 

 cavity is formed between thein by the lateral projection of the alisphenoid. 

 The ligament of the dilitator operculi muscle is inserted on the roughened face 

 of the alisphenoid in this concavity. The very anterior margin of the hyoman- 

 dibula articulates with the posterior edge of the alisphenoid below the ligament 

 insertion. 



The ophthalmic branch of the trigeminus issues from the cranium through a 

 foramen in the wall of the alisphenoid just postero-dorsal to the optic foramen, 

 and extends anteriorly along a ledge above tlie latter. This foramen is the 

 outer end of a short canal which proceeds posteriorly within the alisphenoid 

 and has its cerebral opening near the posterior ventral margin of the internal 

 surface of the bone (Fig. 7). The ophthalmic branch of the facial passes ante- 

 riorly through a more dorsal canal. Although its cerebral opening is just dorsal 

 to the cerebral opening of the ophthalmic branch of the trigeminus, the external 

 opening of the canal is farther forward than the external opening of the latter, 

 and lies just posterior to the point where alisphenoid, frontal and orbitosphe- 

 noid meet. The manner in which these nerves are included within the bone is 

 first seen in the 32 mm. stage (Fig. 4). Up to that time the nerves, after leav- 

 ing the cartilage, extend free across the orbit, but with subsequent development 

 they are gradually enclosed by the ossification of the connective tissue around 

 them in connexion with the alisphenoid cartilage, so that eventually the adult 

 condition is reached. The ventral end of the bone is formed by the ossification 

 of the original membranous wall between the optic and trigeminal nerves 

 (Fig. 32). This method of development of the alisphenoid from cartilage and 

 membrane has been noted in Salmo (Schleip 1903). 



The cerebral surface of the alisphenoid bone is smooth and presents the 

 same relations to the surrounding bones as the external except at its ventral 

 margin, where it is overlapped by the dorsal projection of the suprasphenoid 

 (Fig. 7). The anterior margin of the alisphenoid, where it meets the orbito- 

 sphenoid, has not entirely replaced the underlying cartilage, which still shows 

 through the thin surface lamella. There is probably some cartilage yet remain- 

 ing between the dorsal cap of the alisphenoid and the ventral surface of the 



