in.] THE CRANIAL SKELETON. IO9 



The basi-sphenoid itself, on the other hand, may be hyper- 

 trophied, sending out a long rostrum in front and lateral 

 processes, as is the case in Birds ; and in man's own class it 

 may develop descending and diverging processes, as in the 

 Hedgehog, Centetes, and the Porpoise. In the first two we 

 find a hemispherical depression on the under surface of the 

 sphenoid body, reminding us of the sella turcica on its 

 cranial surface. 



The pre-sphenoid attains not only more distinctness, but a 

 much greater length, in Mammals generally than in man. In 

 all lower forms, however, it is more or less rudimentary or 

 absent : absent, e.g., in Batrachians ; rudimentary in Birds 

 and some Fishes, e.g. the Carp and Salmon. 



The basi-sphenoid may be directly perforated by the in- 

 ternal carotid artery, as is the case in Marsupials ; or the two 

 carotid canals may unite in it, as in Birds. 



The posterior boundary of the sella turcica, with its clinoid 

 processes, is more prominent in man than in almost any other 

 animal, and the depression of the sella is very marked in 

 him. It may be quite indistinct, as in Marsupials and 

 Batrachians. 



The sella may, on the contrary, be a deep and sudden de- 

 pression without any prominent margins directed upwards, as 

 in many Fishes, e.g. the Pike. 



A singular and unexpected function may be performed by 

 the cartilaginous body of the sphenoid, as in the Pike and 

 in very many other Fishes, where it forms part of the roof of 

 a large conical excavation of the basis cranii, in which ex- 

 cavation the muscles of the eyeball take their origin. 



The greater wings of the sphenoid bear in zootomy the 

 name alisphenoid. They form a part of the cranial side-wall 

 which is very generally present in an osseous condition, though 

 not so in Chelonians or Batrachians. Alisphenoids are in- 

 variably developed in all the members of man's class (Mam- 

 malia), and also in Birds and Crocodiles. Usually they take 

 a relatively greater share in the formation of the cranium 

 than in man, whose skull-roof bones are so enormously ex- 

 panded. This is notably the case in Marsupials and some 

 Insectivora, where they extend far back and form the anterior 

 part of the auditory bullae, and in the Kangaroo even touch 

 the paroccipital process. 



The orbital plate of this bone is not developed in Mammals 

 below man's own order, but its homologue is developed in 

 some Birds, e.g. Accipiter and Strixj and it is possible that 



