al 
ei? 
x : 
ON THE VERTEBRATE SKELETON. 189 
of the cranium, that it is still perforated by the third (2b. ¢r) and second 
divisions of the fifth or trigeminal nerve. 5 
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 (fig. 7, 6) 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 alisphenvid 
being pushed forward by the intrusion not only of a large ossified petrosal 
(1s), 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, ér). It may be observed that, con- 
comitantly with this retrogradation of the alisphenoid, the orbito-sphenoid 
(2. 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 (7. 16), 
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 
