MAMMALS 



Burkard (1902) it is a derivative of the maxillary musculature, wliich 

 enters the orbit tlirough the inferior orbital fissure and compensates 

 for the deficiencies of a lateral wall. It is possible that in those 

 animals in which it is well developed it may act as a protrudor muscle 

 by pulling forwards the fascia occupying the fissure and thrusting the 

 eye outwards. 



It is curious that despite the elaborate provision of extra-ocular muscles 

 and their comparative size, the ocular movements of most terrestrial Vertebrates 

 are restricted. •"■ The eye of the elej^hant, for example, is almost immobile despite 

 the fact that the size of its extra-ocular muscles is " stupefying " (Soemmerring, 

 1818), corresponding to the size of the animal rather than to its eye which is 

 relatively small and compares in bulk with that of the ox.- 



497 



Co/mo// 7f/vDou ofO/f/6//^ 



Of Muse. RCTR. BULBI. 



Iat. ffiCTu:, ' ■ ~^^'///f/?Kf 



Fig. 656. — Ax Abnormal Retractor Bulbi Muscle in Man. 



Four muscular bundles run forwards towards the globe, each fusing with 

 a rectus before reaching it. One bundle is innervated by nerve VI (indicated 

 in the figT.u-e), and the others by nerve III (Whitnall, 1911). 



We have seen that among Ampliibians the orbit oj^ens freely into 

 the cavity of the pharynx ; and among most of the lower Vertebrates 

 the post ero-lateral wall remains membranous, opening into the temporal 

 fossa, a commmiication wliich persists in the higher Mammals and man 

 as the inferior orbital fissure, the anterior end of which (in man) may 

 exceptionally encroach upon the lateral wall to form a " spheno- 

 zygomatic fissure" (Tanzi, 1892 ; Duckworth, 1904). The completeness 

 of the orbital bony walls varies considerably^ owing to irregularities in 

 the constituent bones ; the frontal and sphenoid are always jDresent, 

 the ethmoid and the palatine usually do not participate, and accessory 

 ossicles are common (Maggi, 1898). Among the Rodents the orbit is 

 always open, particularly so m the rabbit ; in this animal the floor of 

 the orbit is largely muscular (Davis, 1929). In the elephant and some 

 of the Artiodactyls the orbit is also open and is jiarticularly so among 

 the Carnivores, an adajDtation resembling that seen in lizards and snakes 



p. 692 et seq. 



p. 450. 



S.O.— VOL. I. 



32 



