248 Prof. A. Mucalister. Notes on the Varieties ami 



as Trematosaurus, there are bones in this region probably of the same 

 nature, and comparing these with the suborbital ring in teleost fishes, 

 such as the salmon, the innermost of the series seems so plainly to 

 represent the lachrymal that Professor Parker has affixed this name 

 to it in his figures (fig. 21, " Morphology of the Skull "). 



The lachrymal bone has been inherited by the mammalia in its 

 large facial form, and it participates with its neighbouring bones in 

 the fixity correlated with the setting apart of the jaws for mastica- 

 tion. The upper jaw must be fixed to give attachment to the 

 masseter, which in most of the lower mammals exceeds the temporals 

 (in most Ungulates m : t : : 1 to 2 : 1) ; accordingly the infra-orbital 

 ridge is consolidated into a reversed arch with two piers attached to 

 the skull, of the inner of which the lachrymal forms a solid integral 

 part, and all these marginal elements tend to ankylose into a continu- 

 ous bony ring. 



While these conditions persist the bones retain the same relative 

 place, and we can correlate the changes in structure with changes in 

 function. Among the Primates the approximation to parallelism of 

 the orbital axes diminishes largely the extent of all the elements of the 

 interorbital septum, and is correlated with a strengthened rim of the 

 orbit, especially with an increase in strength in the fronto-jugal pier 

 of the suborbital arch, which new bears the entire masseteric strain. 

 The increase in size of the cranial cavity, together with the 

 diminished interorbital width causes the duct to become entorbital, 

 and it descends obliquely downwards and outwards in the lower 

 forms, nearly vertically in man ; the degree of obliquity depending on 

 the width of the inferior nasal meatus and the narrowness of the 

 interorbital septum. When the duct is obliquely placed the hamulus 

 is large and bony to bear off pressure ; when vertical the hamulus 

 diminishes. While the duct is thus entorbitally displaced, the plane 

 of the teeth is almost completely preorbital, and the pressure of the 

 teeth is borne by the frontal process of the maxilla in front of the 

 orbit, while the masseteric tension is borne by malar bone. Owing to 

 the operation of these three factors, the lachrymal bone sinks into 

 mechanical insignificance. That it is a diminishing bone in man 

 is shown by the early stage at which it attains to its adult condition, 

 as well as by its great variability. 



The Ungulate lachrymal bears a tubercle, which is prominent 

 in the Rhinoceros and Hyrax, and which either lies internal to 

 or separates the canals. This corresponds to the projection of the 

 periosteum, which in man forms the tendo oculi, and from it a contin- 

 uous ridge of periosteum is continued inwards and forwards, from 

 which in Hippopotamus, Rhinoceros, and Ruminants the orbicularis 

 palpebrarnm arises. The small accessory ossicle in the pig is late in 

 ossifying, only showing its first trace of bone after birth. 



