MAMMALS 



495 



(ass) by the presence of accessory muscles accompanying the reflected 

 tendon ; these rejj resent the direction of the original muscle, while the 

 trochlea is situated at the origin of the primitive muscle from the 

 orbital wall (Zimmerl, 1900 ; Mobilio, 1912). In man similar super- 

 numerary fasciculi have been found as an anomaly, or the more 

 primitive arrangement has jjersisted (Ledouble, 1897). The insertions 

 of the obliques vary. In man and the chimpanzee the superior oblique 

 is crossed over by the superior rectus, while the inferior crosses the 

 inferior rectus (Fig. ()50). In the majority of Mammals both obliques 

 are crossed by the recti (Fig. 051) ; in the tiger the recti pierce the 

 obliques (Fig. 052), and in the lion (as in the tortoise) the superior rectus 



Figs. 650 to 633. — The Relation of the Oblique Muscles to the Recti. 



pierces the superior oblicpie, and the inferior oblique pierces the inferior 

 rectus (Fig. 053) (Ottley, 1879 ; Ovio, 1925). 



A retractor bulbi muscle {choanoid muscle, Motais, 1887) occurs 

 in most Mammals; it is particularly developed in Rodents, Ungulates 

 and Sirenians. but is present only in a vestigial form in some 

 monkeys [Macacus) and is absent in some of the higher Primates 

 (F. Smith, 1922 ; Bradley, 1933 ; Winckler. 1933 ; Key-Aberg, 1934). i 

 The muscle arises from the apex of the orbit, and, riuuiing within the 

 muscle-cone, envelops the oj^tic nerve and the posterior part of the 

 globe to be inserted into the sclera behind the recti (Fig. 054). The 

 insertion shows many variations. It may be continuous like the 

 gamopetalous corolla of a flower, or discontinuous with the same 

 general arrangement but in many separate bundles varying in indivi- 

 duals of the same species or even between the two eyes of the same 

 individual, or it may be divided into diverging slips (0 in the sloth-bear, 

 Melursus lahiatus ; 4 in the cat and dog ; 2 in the whale, etc.) (Fig. 

 655). It is sujjplied by nerve VI (Hopkins, 1910), and is usually 

 regarded as a derivative of the lateral rectus (Johnson, 1901 ; Corning, 



^ The muscle is also absent in Cyclostomes, Fishes, O^jhidians and Birds. 



