another and have variable or complex origins and insertions. 

 The origin is the point of anchorage and the insertion is the 

 point moved by the contraction of the muscle. Sometimes 

 these points are purely arbitrary; in a limb the proximal 

 point is always identified as the origin, while in the body 

 the more anterior point is so identified. 



The problems of determining muscle homologies have 

 produced a large literature, and many bulky works such as 

 those of Furbringer are used to identify and discuss these 

 problems. One result of the pioneer work in this field was 

 the conclusion that the musculature of one class could not 

 effectively be compared (i.e. homologized) with that of 

 another. Therefore, it was agreed that muscles of doubtful 

 homology should bear descriptive names derived from their 

 origins and insertions. Muscles that could be homologized 

 between classes would bear the name of the human homolog. 



The view adopted in this text is that the complexity of 

 the existing schemes of muscle nomenclature makes com- 

 parative discussion impossible. It is believed that a single 

 system of names applicable to all classes of tetrapods is de- 

 sirable, and that such a system can be based on the fact 

 that the primitive pre-mammalian line of reptiles (the pely- 

 cosaurs), the primitive ancestors of the living reptiles, and 

 the primitive ancestors of the amphibians were all very 

 much alike: they were creeping animals with a pentadactyl 

 (five digits) hand and foot and a nearly common system of 

 musculature. From this starting point each line evolved sep- 

 arately and independently but retained much of the basic 

 pattern. 



It must be assumed that all muscles in these groups can- 

 not be compared since homologies are not all of the same 

 grade but range from very close, as in the comparison be- 

 tween the closely related species, to rather remote when 

 comparing species at the extremes of a class or when com- 

 paring species of two classes. Within the mammals there is 

 almost as much, or more, variation as there is between se- 

 lected members of different classes. If a single system of 

 names can be used for the mammal, then this system can 

 be extrapolated to other tetrapods. Such a system has the 

 value of indicating the general position and the associations 

 of each muscle. This tends to emphasize any differences, 

 and thus facilitates comparisons. 



Extrinsic muscles of the eye 



The extrinsic muscles of the eye, which rotate the eyeball 

 in its socket, are generally described as six in number. 

 These are innervated by three nerves, the oculomotor, troch- 

 lear, and abducens. The oculomotor innervates the infe- 

 rior oblique, and the internal, superior, and inferior 

 rectus muscles. The superior oblique is innervated by the 

 trochlear nerve, while the abducens innervates the external 

 rectus. 



The variations of eye muscles are interesting particularly 

 in comparing the cyclostomes with the gnathostomes (Figure 



14-4). In the cyclostomes the superior oblique lies behind 

 the eye and tends to rotate the eye counterclockwise in its 

 socket. The recti are not identifiable as superior, internal, 

 and inferior; instead there are two muscles of oculomotor 

 innervation, one of which is in the position of the inferior 

 oblique. The abducens, assuming that this is the homolog of 

 the sixth nerve of gnathostomes, innervates two muscles in 

 the position of external and inferior recti. There is a ques- 

 tion as to whether the inferior rectus is properly named; it 

 has been stated that oculomotor fibers extend out through 

 the abducens to this muscle or that a branch of the oculo- 

 motor sometimes extends to it. 



The condition seen in the lamprey is quite different from 

 that of the usual gnathostome as represented by the shark, 

 Squalus. Noteworthy is the fact that the rectus muscles arise 

 around the optic pedicel not the optic foramen. In Chlamy- 

 doselachus the external rectus is divided into two slips, one of 

 which partly overlaps the superior rectus insertion. In the 

 sawfish a part of the inferior oblique inserts on the orbit 

 margin below the normal insertion. In the rays the entire 

 muscle inserts here. In Carchanas and Miislelus a part of the 

 temporal muscle mass lying behind the eye inserts on the 

 rear margin of a nictitating membrane. The muscle draws 

 this transparent membrane up and back to cover the eye; 

 another part of the muscle mass pulls the upper eyelid 

 down. 



In the actinopterygian fishes the basic pattern of six mus- 

 cles is retained. There is some variation in the course of the 

 oculomotor nerve and its branches to these muscles. The eye 

 muscles of the salmon extend well beyond the margins of 

 the orbit, and their origins lie inside the anterior and poste- 

 rior myodomes (Figures 5-6, 5-7). A posterior myodome is 

 characteristic of most actinopterygians. 



In the dipnoan Protoplerus there are the usual six muscles. 

 The inferior rectus and internal rectus are close together 

 and only partly differentiated. The external, inferior, and 

 internal rectus arise together below the optic nerve, while 

 the superior rectus arises more posteriorly on the connective 

 tissue overlying the optic nerve. Latimeria also has the infe- 

 rior and internal recti closely joined. 



In the Amphibia the pattern is the same as in the fish, 

 but the external rectus has divided to form a retractor bulbi 

 and an external rectus. The name external rectus is thus 

 used in two senses. 



In the reptile (lizard) the pattern is comparable. The re- 

 tractor bulbi is now divided into several sips. There are 

 usually two of these: an upper bursalis and a lower "re- 

 tractor bulbi." Both of these take origin inside of a posterior 

 myodome in Sphenodon and squamates. In Varanus there 

 are three retractors: two dorsal bursalis muscles and a single 

 ventral retractor bulbi. 



The bird is quite distinct as compared with the reptile. 

 The basic pattern of six muscles is the same but the re- 

 tractor bulbi has been modified into a quadratus and a 

 pyramidalis. The pyramidalis has a long tendon circling 



422 • THE MUSCULAR SYSTEM 



