312 
DR. CLELAND ON THE RELATIONS 
The real constitution of the facial segment will become evident if we consider the 
series of appearances assumed by it, passing upwards from the fish. In the fish the 
intermaxillaries and maxillaries hanging downwards form a sufiiciently e\fident incomplete 
hfemal arch ; while the ideal cylinder formed by the series of neural arches is brought 
to a close in front by the nasal, which rests, like the nasals in the other classes, upon 
the cartilage in connexion with which the centra are developed ; and when the centrum 
of this sclerotome is represented by a plate of bone, it is a process of the nasal which 
represents it. Obviously the nostril of the fish lies behind the facial segment, for both 
the intermaxillaries and maxillaries lie in front of the nostril (fig. 31). 
The facial segment of the Frog differs from that of the fish in that the maxillaries are 
directed horizontally backwards, and come in contact with bones behind ; and in that 
the centrum of the segment is represented by a process derived, not from the nasals, but 
from the intermaxillaries. Further, the nasals of the Frog differ from the nasal of the 
fish in being expanded to protect the nostrils, like those of most reptilia, bu’ds, and 
mammals. The exact signification of this expansion we shall consider anon. 
The relation of the nostril of the Frog to the facial segment is the same as in the 
fish, but inasmuch as it communicates with the mucous surface, and this communication 
lies between the maxillary and the palatal, we can now see distinctly that the nostril 
is a passage lying between the segments to which these bones belong, — what has been 
named by Professor Goodsie a “ metasomatomic” opening*. 
In lizards, serpents, and birds the facial sclerotome is constituted as in the Frog ; and 
the haemal arch has still, as in the Frog, its distal extremities directed backwards instead 
of downwards. According to this view the superior mesial processes of the intermaxil- 
laries are to be considered morphologically as projecting not so much upwards as back- 
wards, and as lying above the nostrils, while the haemal arch lies in front of the 
nostrils. 
Lastly, in the mammalia, by the union of the palate plates of the maxillaries the 
haemal arch is completed, and the ring of bone which surrounds the incisive foramina is 
the anterior extremity of the ideal cylinder formed by the series of haemal arches 
(fig. 27). This is best conceived of by looking at this ring in the Hare or the Eabbit. 
There we see the single incisive foramen bounded in the middle line in front by the 
mesial-palatine processes of the intermaxillaries, while extending backwards Aom them 
is the haemal arch formed by intermaxillaries and maxillaries exactly as in other classes, 
except that it is complete. 
The fact that in certain mammals the intermaxillaries limit the anterior extremity of 
the septal cartilage as much as they do in birds and lizards, embracing it at a point 
morphologically anterior to the nostrils, appears to me to prove that this is the true 
explanation of the facial sclerotome in the mammalia, and that the nostrils throughout 
the vertebrata are intersegmental openings lying between the two most anterior segments 
of the skull. 
* To Professor GrooDsia we are also iudebted for the terms “ sclerotome, myotome,” &c. and other addi- 
tions to explicit morphological nomenclature. 
