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Observations on the muscles of the eye of the bowhead whale,Balaena mysticetus

2000; Wiley; Volume: 259; Issue: 2 Linguagem: Inglês

10.1002/(sici)1097-0185(20000601)259

1097-0185

Qian Zhu, Daniel J. Hillmann, William G. Henk,

Ichthyology and Marine Biology

The Anatomical RecordVolume 259, Issue 2 p. 189-204 ArticleFree Access Observations on the muscles of the eye of the bowhead whale, Balaena mysticetus Qian Zhu, Qian Zhu Department of Marine Biology, Shandong University, Weihai Campus, Weihai 264209, Shandong Province, People's Republic of ChinaSearch for more papers by this authorDaniel John Hillmann, Corresponding Author Daniel John Hillmann dhillmann@mail.vetmed.lsu.edu Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803; Fax: 225-346-3329Search for more papers by this authorWilliam George Henk, William George Henk Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803Search for more papers by this author Qian Zhu, Qian Zhu Department of Marine Biology, Shandong University, Weihai Campus, Weihai 264209, Shandong Province, People's Republic of ChinaSearch for more papers by this authorDaniel John Hillmann, Corresponding Author Daniel John Hillmann dhillmann@mail.vetmed.lsu.edu Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803; Fax: 225-346-3329Search for more papers by this authorWilliam George Henk, William George Henk Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803Search for more papers by this author First published: 16 May 2000 https://doi.org/10.1002/(SICI)1097-0185(20000601)259:2 3.0.CO;2-HCitations: 17AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The muscles of the eyelids and the extraocular muscles of mysticete whales are poorly described for a variety of reasons, including considerable difficulty in obtaining specimens. Our objective is to provide such a description for the bowhead whale, Balaena mysticetus. This study has examined the gross anatomy of the region in six specimens (five adults, one fetus) of the bowhead whale. Results show that the muscles associated with the eye are well developed and possess several distinctive features. For example, precise limits of each extraocular muscle are difficult to determine along their entire length because these muscles intermingle with one another near their insertion. Furthermore, some fibers from these muscles (except the retractor bulbi) also insert into the eyelids. Pulling on these muscles to simulate contraction results in movement of the eyelids and suggests a role for these muscles in palpebral retraction. Insertion of a large levator palpebrae superioris muscle into the upper eyelid further enhances opening of the palpebral fissure. Another unusual feature is the presence of tunnel-like structures that redirect the dorsal and ventral oblique muscles. The dorsal oblique muscle is redirected caudally about 90 degrees, then directed medially by another 90 degrees. These directional changes are accomplished via a connective tissue tunnel derived in part from the fibrous connective tissue of the dorsal rectus and the levator palpebrae superioris muscles. In most terrestrial mammals, a similar change in direction is accomplished by a cartilaginous trochlea. The ventral oblique muscle originates via a slender tendon from the frontal bone and undergoes a similar radical change in direction. Its tendon of insertion undergoes about a 90-degree change in direction that is accomplished through a tunnel-like structure derived from fibrous connective tissue of the ventral rectus muscle. Based on the morphology of the musculature presented, it is likely that the eyeballs and eyelids of the bowhead whale are quite mobile and appear capable of complex movement. The possibility of retraction and protrusion of the eyeball is discussed. Anat Rec 259:189–204, 2000. © 2000 Wiley-Liss, Inc. The muscles of the eyes of a rather small number of cetacean species have been investigated. Tyson (1680) described the orbit and eye musculature in Phocoena sp., noting the large retractor bulbi muscle that he believed was involved in altering the shape of the eyeball during accommodation. Hunter (1787) described the general arrangement of the extraocular muscles in cetaceans, and identified the retractor bulbi and four recti muscles. Mayer (1852) mentioned that "in the baleen whale (Balaena mysticetus) the six regular eye muscles are completely missing and the only remaining muscle is the large, strong, and undivided musculus retractor bulbi." Weber (1886) described the extraocular muscles and their innervation in several odontocetes and mysticetes and considered their function. Hosokawa (1951) described the extraocular muscles, including the arrangement of the muscles and their innervation in the sei whale (Balaenoptera borealis), blue whale (B. musculus), fin whale (B. physalus), sperm whale (Physeter catodon), pilot whale (Globicephalus melas), and a dolphin (Prodelphinus caeruleo-albus). Dubielzig and Aguirre (1980a,b, 1981) noted the tendons of the recti muscles in the bowhead whale (B. mysticetus). Haldiman and Abdelbaki, et al. (1982), Abdelbaki and Haldiman (1986) and Haldiman and Tarpley (1993) described the extraocular muscles of the bowhead whale as being well developed, and "arranged in a cone consisting of many thin, small, fusiform muscle bundles united by sheets of white fibrous connective tissue." Zhu (1996, 1998) likewise, mentioned the muscles of the eye of the bowhead whale. A more detailed description of the muscles of the eye of B. mysticetus is presented in this paper. From this brief review, it is evident that the eye muscles of cetaceans have been poorly studied. This is in part due to various limitations including the availability of suitable specimens. Specimens used in many previous studies were unfit for accurate description of the eye muscles because the eyeball was often separated from the extraocular structures. The objective of this article is to describe the gross anatomy of the muscles of the eyelids and the extraocular muscles in the bowhead whale. The observations presented are based on in situ dissections of six bowhead whales (including one fetus), a large sample considering the difficulty in obtaining such specimens. MATERIALS AND METHODS The bowhead whale is of great nutritional and cultural importance to the Eskimo people of northern and western Alaska. The Eskimo hunters allow specimens to be collected from their subsistence harvest for scientific research. All the specimens used in this study were collected near Barrow, Alaska. Five adults and one fetus were studied (Table 1). Specimens from freshly harvested animals were quickly frozen or fixed in 10% neutral-buffered formalin at either the harvest site or the nearby Arctic Research Facility of the North Slope Borough in Barrow. Table 1. Specimens used in this study Whale identification No. Sex Body length (m) Right or left eye 93B20F Female 1.9 (fetus) Right and left eyes 95B11 Male 13 Right eye 95B16 Male 14.1 Right eye 95B17 Female 7.5 Left eye 95B18 Female 7.6 Right eye 96B1 Female 8.5 Left eye 96B2 Female 7.6 Right eye Relevant structures were photographed to provide a permanent record. Once the dissections were completed, the gross anatomical structures were also recorded using 8mm-format videotape. Magnetic resonance imaging (MRI) was conducted on one large block of tissue containing the eyeball and its associated structures. This approach permitted the nondestructive in situ localization of all structures associated with the eye. This specimen was initially preserved by freezing in Barrow and subsequently thawed in formalin at Louisiana State University. Dissections were performed using standard anatomical instruments at the Arctic Research Facility in Barrow, the Department of Veterinary Anatomy and Cell Biology, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana, and the Institute of Oceanology, Chinese Academy of Sciences, Qingdao, People's Republic of China. RESULTS Terminology In some mammals, such as the human and the dog, the eyes face forward and the positional terminology as applied to the eye is easy to understand. For example, the left eye in human faces directly forward and can therefore be said to have a "top" (dorsal aspect), a "bottom" (ventral aspect), a "left side" (lateral aspect), a "right side" (medial aspect), a "front" (anterior aspect) through which light enters, and a "rear" (posterior aspect) from which the optic nerve leaves. In the bowhead whale the eyes are positioned so that each eye faces to the "side" or laterally. That is, the left eye "looks" almost directly to the left and the right eye "looks" almost directly to the right. Orientation of the eye in the bowhead whale presents problems when applying the anatomical terms of position and direction. In the human and in the dog, light enters through the "front" or anterior aspect of the eye, but in the bowhead whale the "front" or external surface of the eye is positioned laterally with respect to the head. Thus, light actually enters the eye from a lateral direction (Figs. 1, 2). Figure 1Open in figure viewerPowerPoint Semischematic illustration showing the orientation of the left eye of the bowhead whale in relation to the skull viewed from the dorsal aspect. Enlargement shows the eyeball and its associated structures located beneath the eyelids and within the periorbital cone. Observe that the eyeball "looks" laterally. Figure 2Open in figure viewerPowerPoint Semischematic illustration of a longitudinal section of the left eye of the bowhead whale viewed from the caudal aspect. Terms of position and direction have been included to assist the viewer with orientation of the eyeball and associated structures. Our use of anatomical nomenclature for the muscles of the eye of the bowhead whale follows Nomina Anatomica Veterinaria (1994) (N.A.V.). This nomenclature is appropriate from a positional standpoint, where the eyes are directed forward. Since that is not the case in the bowhead whale, some confusion may result. For example, the lateral rectus muscle is actually positioned caudal to the eyeball. Simply altering the anatomical nomenclature to be more positionally accurate would not solve this problem. To do so would cause more confusion over the embryonic origin and phylogenetic homology of the muscles and structures involved. We have chosen to use the anatomical nomenclature (N.A.V.) despite the misleading positional information contained in the muscle names. The difference of position and direction of the eyes of the human and the dog compared with the bowhead whale is listed in Table 2. Table 2. Terms of position and direction of the right eye of the human and the dog compared with the bowhead whale Human and dog Bowhead whale Eye "faces" forward Eye "faces" to the right Top or dorsal aspect of the eye Same Bottom or ventral aspect of the eye Same Left side or medial aspect Rostral aspect Right side or lateral aspect Caudal aspect Front or anterior aspect, through which light enters Lateral aspect Rear or posterior aspect, from which optic nerve leaves Medial aspect Dorsal or superior rectus muscle (dorsal aspect) Dorsal rectus muscle (dorsal aspect) Ventral or inferior rectus muscle (ventral aspect) Ventral rectus muscle (ventral aspect) Medial rectus muscle (medial aspect) Medial rectus muscle (rostral aspect) Lateral rectus muscle (lateral aspect) Lateral rectus muscle (caudal aspect) Dorsal or superior oblique muscle (dorsomedial aspect) Dorsal oblique muscle (dorsorostral aspect) Ventral or inferior oblique muscle (ventrolateral aspect) Ventral oblique muscle (ventrocaudal aspect) Retractor bulbi muscle (posterior aspect) Retractor bulbi muscle (medial aspect) Medial canthus (medial aspect) Medial canthus (rostral aspect) Lateral canthus (lateral aspect) Lateral canthus (caudal aspect) Muscles of the Eyelids The muscles of the eyelids of the bowhead whale consist of the M. orbicularis oculi, M. levator anguli oculi medialis, M. retractor anguli oculi lateralis, and the M. levator palpebrae superioris. These muscles are well developed and function to close and open the eyelids. M. orbicularis oculi The orbicularis oculi muscle is a very thick sphincter-like muscle. It is well developed and located deep within the tissue of the eyelids (Fig. 3). Its fibers completely surround the palpebral fissure. Chief attachment of these fibers is to the overlying dermis of the skin of the eyelids. Some fibers are intermingled with those of the levator anguli oculi medialis and the retractor anguli oculi lateralis. The probable function of the orbicularis oculi muscle in the bowhead whale is to close the eyelids. Figure 3Open in figure viewerPowerPoint Superficial structures of the dissected left eye of bowhead whale 95B17. The dissection is viewed from the dorsolateral aspect of the head in the region of the orbit. Observe the cleaned frontal bone depicting the bony orbital margin. The orbicularis oculi muscle is well developed and is located within the connective tissue of the eyelids. It is intimately attached to the overlying dermis of the upper and lower eyelids (the latter having been removed to expose the underlying muscle). Observe that some of the fibers of the orbicularis oculi muscle intermingle with the retractor muscle of the lateral angle of the eye. M. levator anguli oculi medialis and M. retractor anguli oculi lateralis The levator anguli oculi medialis and retractor anguli oculi lateralis muscles are well developed. The levator anguli oculi medialis originates from the maxillary, lacrimal, and zygomatic bones. Its fibers radiate distally (laterally with respect to the head) into the eye capsule, blending most distally with the orbicularis oculi at the medial canthus of the eye (Fig. 4; see also Fig. 8). Near its origin, the levator anguli oculi medialis is thick, becoming thinner near the margin of the lid. The probable function of the levator anguli oculi medialis is to increase the rostral to caudal length of the palpebral fissure by fixing the medial canthus. Figure 4Open in figure viewerPowerPoint Superficial structures of the dissected left eye of bowhead whale 95B17. The dissection is viewed from the rostral aspect of the head in the region of the orbit. Observe the cleaned maxillary, lacrimal, and zygomatic bones depicting the bony orbital margin. Note that the eye and associated extraocular muscles are located well outside (distolateral to) the bony orbit. The orbicularis oculi muscle is well developed and is located within the connective tissue of the eyelids. It is intimately attached to the overlying dermis/epidermis of the upper and lower eyelids (the latter having been removed to expose the underlying muscle). Observe the levator muscle of the medial angle of the eye originating within the periosteum of the maxillary and lacrimal bones. From its origin, it extends distally, intermingling with fibers of the orbicularis oculi muscle, before attaching to the medial angle of the eye. The retractor anguli oculi lateralis originates from the frontal bone and radiates distally (laterally with respect to the head) into the eye capsule. A portion of the retractor anguli oculi lateralis continues distally, blending with the orbicularis oculi at the lateral canthus of the eye (Fig. 5). Near its origin, the retractor anguli oculi lateralis muscle is thick. It becomes thinner near the margin of the eyelid, where it blends with the orbicularis oculi. The probable function of the retractor anguli oculi lateralis is to increase the rostral to caudal length of the palpebral fissure by retracting the lateral angle of the eye. Figure 5Open in figure viewerPowerPoint Superficial structures of the dissected left eye of bowhead whale 95B17. The dissection is viewed from the caudal aspect of the head in the region of the orbit. Observe the relative size and development of the retractor muscle of the lateral angle of the eye compared with the other extraocular muscles. Note that it arises from the periosteum covering the frontal bone. It extends distolaterally to insert at the lateral angle of the eye. Muscle fibers of the retractor muscle intermingle with those of the orbicularis oculi muscle. M. levator palpebrae superioris The levator palpebrae superioris (Fig. 6) is located between the dorsal rectus and the dorsal oblique muscles. In bowhead whale 95B11, the total length (from origin to insertion) of the levator palpebrae superioris is 64 cm, and its greatest width is 3.9 cm. Its tendon of origin is fused with that of the dorsal oblique, where they reach the orbital apex along with tendons of the other extraocular muscles. Its broad tendon of insertion ends in the upper eyelid. Near its insertion, fibers of the levator palpebrae superioris intermingle with those of the dorsal rectus. Where the latter conjoined fibers insert into the upper eyelid, the levator palpebrae superioris continues, sending its fibers deeper into the structure of the upper eyelid. Figure 6Open in figure viewerPowerPoint Extraocular muscles of the dissected left eye of bowhead whale 95B17. The dissection is viewed from the rostral aspect of the head in the region of the orbit. Observe the frontal bone where it forms the dorsal margin of the bony orbit. It lies dorsal to the extraocular muscles throughout much of their internal course. The well-developed levator muscle of the upper eyelid is located between the dorsal rectus and dorsal oblique muscles. Internally (at their origins), the levator of the upper eyelid and dorsal oblique muscles share a common tendinous origin with many of the other extraocular muscles. Observe the well-developed ventral oblique muscle. It originates from the internal surface of the frontal bone, coursing around the rostromedial aspect of the eyeball, finally inserting on that structure. In some mammals, when the levator palpebrae superioris contracts, the upper eyelid is elevated (Getty, 1975). The function of the levator palpebrae superioris in the bowhead whale is the same as in those mammals. Extraocular Muscles It is well known that mammals have seven extraocular muscles. These muscles consist of the M. rectus dorsalis, M. rectus ventralis, M. rectus medialis, M. rectus lateralis, M. obliquus dorsalis, M. obliquus ventralis, and the M. retractor bulbi. The extraocular muscles provide ocular mobility. The extraocular muscles of the bowhead whale are very well developed and are arranged in a cone-like fashion with each muscle consisting of many small, fusiform muscle bundles united by fibrous connective tissue (Fig. 7). This cone of muscles tapers from the eyeball where it extends proximally toward the apex of the orbit (Fig. 1). Sling-like fibrous tissue forms the "floor" of the passageway for the cone of muscles while its "roof" is formed by the deeply grooved ventral surface of the overlying frontal bone. Figure 7Open in figure viewerPowerPoint Magnetic resonance image (transverse section) of the extraocular muscles of bowhead whale 96B2, imaged at the level of the bony orbital rim (temporal, frontal, maxillary, lacrimal, and zygomatic bones). Dashed line superimposed on schematic insert represents level of transverse section. Note the large amount of fat contained within the bony orbit and periorbital fascia. In transverse section, the extraocular muscles appear as elongated ovals. The retractor bulbi muscle, along with a small amount of fat, completely surrounds the optic nerve. LPS = M. levator palpebrae superioris; DO − M. obliquus dorsalis; MR − M. rectus medialis; LR − M. rectus lateralis; DR − M. rectus dorsalis; VR − M. rectus ventralis; RB − M. retractor bulbi, ON − N. opticus. The extraocular muscles are so well developed that there is a great disparity between the eyeball size and the size of the extraocular muscles when compared with other mammals, such as the horse, ox, cat, and human. Within the periorbital cone of connective tissue there is also a large amount of fat that may serve to cushion structures during movement of the extraocular muscles. The highly vascularized fat may facilitate temperature changes such as when the eyelids are open and exposed to cold water. The extraocular muscles of the bowhead whale are not as "dark" in color as the nearly "black" myoglobin-rich epaxial muscles. The dorsal rectus, ventral rectus, lateral rectus, medial rectus, dorsal oblique, and ventral oblique muscles are pinkish red in color and very similar in color to the major body muscles of terrestrial mammals. The color of the retractor bulbi is reddish purple and is clearly different from the other muscles. The reason for these differences in color is not clear and awaits further study. Compared with many mammals, the exact limits (margins) of the extraocular muscles in the bowhead whale are very difficult to define. They tend to merge with one another at their insertions, collectively forming an almost complete circumorbital cone that attaches to the eyeball (see Fig. 10). In the bowhead whale, some fibers of each extraocular muscle (except the retractor bulbi) insert into the eyelids (palpebral insertion) (Fig. 8). Simulated contraction of the extraocular muscles during dissection of the eye of the bowhead whale results in movement of the eyelids, suggesting that these muscles also play a role in retracting the eyelids. Figure 8Open in figure viewerPowerPoint Horizontal, somewhat oblique, magnetic resonance image through the eye and related structures of bowhead whale 96B2. Majority of structures imaged are normally located beyond (distal) to the bony orbit. Note the continuity of the fibers of the elevator and retractor muscles of the respective medial and lateral angles of the eye with the oribicular muscle of the eye. Observe the numerous palpebral insertions of both the ventral and medial rectus muscles. In the bowhead whale, contraction of these extraocular muscles plays a role in retracting the eyelids, increasing the palpebral aperture. LAOM − M. levator anguli oculi medialis; RAOL − M. retractor anguli oculi lateralis; MR − M. rectus medialis; LR − M. rectus lateralis; RB − M. retractor bulbi; orbicularis oculi − M. orbicularis oculi. The four recti and the two oblique muscles are inserted at different points on the eyeball. The distal portion of each of these six muscles consists of a tendon which spreads, fan-like, to insert onto the sclera. Unlike many mammals, the scleral insertions of these extraocular muscles form an almost complete circumorbital cone that surrounds the eyeball (Fig. 9, large dashed line). Widths of the extraocular muscle tendons (except the retractor bulbi) at points of insertion on the sclera in bowhead whale 95B16 are presented in Table 3. Total length (from origin to insertion) and the greatest width of the extraocular muscles in bowhead whale 95B11 are presented in Table 4. Figure 9Open in figure viewerPowerPoint Extraocular muscles of the right eye of bowhead whale 95B11 viewed from the internal aspect (e.g., "looking" distolateral from within the bony orbit). This "fresh" (unfixed) dissection illustrates the relative thickness of the extraocular muscles compared to the size of the ocular bulb (dotted outline). Observe that many of the extraocular muscles insert on the sclera via numerous, small tendons lying on the deep face of each muscle belly. This unique arrangement of muscle bellies and tendons forms an almost complete cone surrounding the ocular bulb. Table 3. Widths of the extraocular muscle tendons (except the retractor bulbi) at points of insertion on the sclera in bowhead whale 95B16 Extraocular muscles Width of tendon (cm) Ventral rectus muscle 6.8 Dorsal rectus muscle 2.1 Medial rectus muscle 2.0 Lateral rectus muscle 3.4 Dorsal oblique muscle 2.0 Ventral oblique muscle 3.4 Table 4. Total length (from origin to insertion) and greatest width of the extraocular muscles in bowhead whale 95B11 Extraocular muscles Total length (cm) Greatest width (cm) Dorsal rectus muscle 74 7.2 Ventral rectus muscle 71 10.1 Medial rectus muscle 63 3.9 Lateral rectus muscle 74 6.8 Dorsal oblique muscle 74 7.2 Ventral oblique muscle 39 4.6 Retractor bulbi muscle 60.5 6.8 The dorsal rectus, ventral rectus, medial rectus, and lateral rectus muscles originate from periosteal connective tissue surrounding the origin of the orbital apex deep within the bony orbit. They pass distally and insert onto both the sclera near the limbus (scleral insertion) and the eyelids (palpebral insertion). Near its origin, the width of each extraocular muscle's tendon at the fusion point (point where the tendon of one extraocular muscle is fused with the tendon of another extraocular muscle) in bowhead whale 95B18 is presented in Table 5. Table 5. Width of each extraocular muscle tendon at the fusion point with another extraocular muscle near the origin in bowhead whale 95B18 Extraocular muscles Tendon width at the fusion point (cm) Retractor bulbi muscle It is still muscle at its fusion point Ventral rectus muscle 2.9 Dorsal rectus muscle 0.5 Medial rectus muscle 2.5 Lateral rectus muscle 2.1 Dorsal oblique muscle 0.5 Ventral oblique muscle It originates on the frontal bone The distance from the limbus to the point of scleral insertion of each extraocular muscle (except the retractor bulbi muscle) in bowhead whale 95B11 is presented in Table 6. Table 6. Distance from the limbus to the point of scleral insertion of each extraocular muscle (except the retractor bulbi muscle) in bowhead whale 95B11 Extraocular muscle Distance from limbus to scleral insertion (cm) Ventral rectus muscle 2.2 Dorsal rectus muscle 1.8 Medial rectus muscle 1.6 Lateral rectus muscle 2.5 Dorsal oblique muscle 2.2 Ventral oblique muscle 2.5 M. rectus dorsalis The dorsal rectus muscle originates from within the orbital apex and extends distally toward the eyeball within the orbit. Unlike most mammals, one insertion of the dorsal rectus continues distally into the upper eyelid (palpebral insertion) (Fig. 10). A second, deeper insertion attaches on to the dorsal surface of the sclera (1.8 cm behind the limbus in bowhead whale 95B11). The total length (from origin to insertion) is 74 cm, and its greatest width is 7.2 cm in bowhead whale 95B11. Figure 10Open in figure viewerPowerPoint Extraocular muscles of the left eye of bowhead whale 95B17 viewed from the internal aspect (e.g., "looking" distolateral from within the bony orbit). This "fresh" (unfixed) dissection illustrates the relative thickness of the extraocular muscles compared to the size of the ocular bulb (dotted outline). Observe that the straight muscles of the eye (e.g., dorsal, ventral, lateral, and medial rectus mm.) insert on the sclera via relatively wide flat tendons. In addition, each straight muscle sends tendons into the overlying upper and lower eyelids, where they form a thickened "ring" surrounding the eyeball. Since the dorsal rectus attaches both on the dorsal surface of the sclera and into the upper eyelid, contraction of this muscle would align the optical axis of the eyeball in an upward (dorsal) direction and result in an upward movement of the upper eyelid. M. rectus ventralis The ventral rectus (Fig. 11) originates from the ventral aspect of the orbital apex. It is a very broad muscle that extends distally and inserts on the ventral surface of the sclera (2.2 cm behind the limbus in bowhead whale 95B11). A second insertion of the ventral rectus continues distally into the lower eyelid. The ventral rectus is the best developed of the extraocular muscles. The total length (from origin to insertion) of the ventral rectus is 71 cm, and its greatest width is 10.1 cm in bowhead whale 95B11. Figure 11Open in figure viewerPowerPoint Extraocular muscles of the dissected left eye of bowhead whale 95B17. The dissection is viewed from the ventral aspect of the head in the region of the orbit. Observe the cleaned frontal bone where it forms the margin of the bony orbit. To expose the extraocular muscles from the ventral aspect, the strong periorbital fascia has been incised and reflected. Note the distinct conical nature of the extraocular muscles, where they lie within the bony orbit. The ventral rectus muscle becomes broad and relatively thick as it approaches the eyeball. Deep to the muscle belly, it releases a flattened tendon that inserts on the sclera of the eyeball. Superficially, it releases tendons that blend with the overlying connective tissue, orbicularis muscle, and dermis of the lower eyelid. The ventral rectus is attached on the ventral surface (below) of the sclera, while the dorsal rectus is attached on the dorsal surface (above) o