ON THE VERTEBRATE SKELETON. 



\H<J 



of the cranium, that it is still perforated by the third (ib. tr) and second 

 divisions of the fifth or trigeminal nerve. 



In tracing the alisphenoid downwards through the mammalian series, we can- 

 not but be impressed with the conviction of its true character and importance 

 as an essential part of the cranium, from its constancy in the formation of its 

 walls, and by observing that, whilst the share which the squamosal takes in them 

 progressively decreases, — until in the sheep, for example, it is quite excluded 



Fig. 7. 



Vertical longitudinal section of the cranium of a sheep (Ovis Aries). 



from the cranial cavity, — that of the alisphenoid (rig. 7, e) increases as the 

 cavity itself diminishes in size ; and, further, that this increase is not accom- 

 panied with any material change in the relative size of the alisphenoid to the 

 basisphenoid. The share which the alisphenoid takes in forming the ante- 

 rior boundary of the otocrane increases; as does also the extent of its supe- 

 rior connections, especially of that with the parietal (7). It is important, 

 in tracing these modifications, to note, also, the change in the relative position 

 of the foramen ovale in the mammalian series. In Man the foramen ovale 

 (fig. 6, tr) is close to the hinder border of the alisphenoid ; and in some 

 quadrumanes the third division of the fifth escapes through a notch in the 

 same border. This position of the foramen ovale relates to the alisphenoid 

 being pushed forward by the intrusion not only of a large ossified petrosal 

 (i6), but of a still larger squamosal (27). In the sheep, however, the fora- 

 men ovale is no longer at the posterior margin ; but, the alisphenoid, having 

 retrograded by the recession of the squamosal towards its more normal ex- 

 terior position in the vertebrate series, the third division of the trigeminal 

 now perforates its middle part (fig. 7» tr). It may be observed that, con- 

 comitantly with this retrogradation of the alisphenoid, the orbito-sphenoid 

 (ib. 10) acquires larger proportional dimensions than in Man (fig. 6, 10). 



In the bird the alisphenoid (fig. 8, 6) is recognizable by the repetition of 

 the connections which it presented in the sheep; the squamosal being quite 

 excluded from the cranial parietes, and, indeed, never again presenting itself 

 in the capacity of a cranial bone in any of the oviparous vertebrates. The 

 alisphenoid (fig. 23, 6) is in contact posteriorly with the petrosal (ib. ie), 

 which soon becomes anchylosed with it, as well as with the exoccipital (2), 

 mastoid (s), and other bones forming the cavity for its reception, in all birds. 

 The alisphenoid further manifests its true homology in the bird by its other 

 constant character of transmitting the third and also the second or maxillary 

 division of the trigeminal nerve ; which divisions, in the young ostrich, I 



