13] THE SKULL OF AM lURUS— KINDRED 13 



This same author claims that the cartilaginous bar, which I, following 

 Sewertzofif , have called the aUsphenoid cartilage, is not homologous with the 

 alisphenoid cartilage of the MammaUa and therefore cannot be so named. 

 He says that the ala temporaUs in the mammalian cranium is a new formation, 

 homologous with the process basipterygoideus of the LacertiUan cranium, 

 which arises from the procartilage cells aroimd the anterior end of the palato- 

 pterygoid cartilage. 



On comparison with the alisphenoid of mammals this homology falls to 

 the ground if the relations of nerve and cartilage are used as the criteria for >^ 

 homologies of the chondrocranial parts. Throughout all of the lower Gnath- 

 ostome groups the posterior end of this cartilage is connected with the 

 development of the foramina for the passage of the branches of the trigeminus, 

 but when such a cartilage appears in the mammals, the above associations 

 are discounted and the cartilage is compared to an outside formation. In 

 denying the homology, Gaupp takes this question into consideration, but 

 maintains that the nerve relations are secondary and that nerves go through 

 the ala temporaUs because it replaces the original wall of this region. Assiun- 

 ing this to be the case, he states that anybody who relies upon the passage of 

 nerves for their criteria of homology of cartilaginous parts is sure to err (1902). 

 It is a settled fact, however, that>lLre nerves' constant throughout the vertebrate ^' 

 series and that they precede the cartilage in both ontogeny and phylogeny. 

 Therefore, any homology which is made with these as a basis is sure to have a 

 landmark which varies less than the parts of any other organic system, such 

 as the blood vessels or muscles. 



The trabeculae cranii are flat and acute on both inner and outer edges, ^ 

 becoming narrower anteriorly before imiting with the ahsphenoid cartilages. 

 Their posterior ends are fused with the anterior parachordalia and with them ' 

 form the lateral margins of the fenestra hypophyseos and the fenestra basi- 

 cranii anterior (Figs. 2, 14). The anterior ends are fused to each other median 

 to their union with the ahsphenoid cartilages and form the posterior margin 

 of the ethmoid plate which has been discussed. There is no trabecula com- 

 munis such as occurs in Salmo, the cranium being distinctly platybasic. An, '^ 

 internal carotid artery approaches the trabecula of its side ventrally, and 

 enters the cranial cavity through the inner edge in about the middle region 

 of the orbit (Fig. 2), and proceeds anteriorly in the membranous wall of the 

 orbit. Each artery sends a branch along the dorsal surface of the optic nerve 

 and then unites lateral to the cerebral hemispheres with an internal branch 

 from the external carotid. 



The rectus eye muscles and a ligament to the pterygoid cartilage are ' 

 attached to the lateral surface of each trabecula in the posterior part of the 

 orbit. There is no trace of a myodome in this or the later stages. The oblique 

 eye muscles are inserted on the trabeculae below the orbital foramen. 



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