Tag Archives: functional anatomy

Walk Like a Sloth–lesson 12: the femur

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

Getting Oriented

 

The femur or thigh bone is the largest bone in the sloth’s body and one of the most unusual bones of any animal owing to to its tremendous width and eccentric cross-section.  eccentricThe round head fits into a cup-shaped socket in the pelvis or hip called the acetabulum, while the far or distal end forms the knee joint, articulating with both the tibia or shin bone and patella or knee cap. The head of the femur points upward, about 35° below vertical, and angles forward about 45°, matching the backward-pointing hip sock.  This gives Megalonyx a unique knees-wide-apart stance.  (McDonald, 1977) The distal (down) end of the bone has a shallow wide depression in the center of one side–that’s the trochlea or patellar groove, where the patella articulates.  That’s anterior (forward) obviously, so this femur came from the sloth’s left leg.

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Walk Like a Sloth–lesson 11: the toes

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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Like you, and unlike tree sloths and most other ground sloths, Megalonyx had five (5) toes on each foot.  Your toes, like your fingers, have three (3) phalanges (segments) each, except your big toe and thumb which each have just two (2) phalanges. Megalonyx is the same, only the middle toe is the big toe, and has two (2) bones.  In Megalonyx the proximal phalanx (the bone closer to the body) and medial phalanx (middle bone) of the 3rd toe are fused, so completely that all traces that they started out as two separate bones have vanished. What looks like the proximal phalanx is actually the third metatarsal.

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Walk Like a Sloth–more things to do with the scapula

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magnifyingglassLook closer

The glenoid cavity is pear-shaped, wider at the bottom and narrowing at the superior end. The oval shape is generally believed to confer a greater range of motion versus the narrower sockets of quadrupeds.  (Aielo and Dean, 1990)  Note the glenoid notch midway up the ventral rim of the socket of the adult’s bone.  About half (55%) of humans have them too. (Prescher and Klümpen, 1997) Scientists disagree about their purpose.  The ventral surface of the scapula or subscapular fossa gives origin to the broad subscapularis muscle, part of the rotator cuff  quartet, which gathers as a wide tendon wrapping around the glenoid cavity to insert on the lesser tubercle of the humerus.  The notch may simply form due to the pressure of a tendon and subsequent atrophy of the bone. (ibid) Alternatively, it may provide extra surface area for anchoring the inferior glenohumeral ligament, one of ligaments surrounding the cavity, important for stabilizing the humerus head (Miles, 1997).

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Walk Like a Sloth–lesson 10: the scapulae

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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The scapula or shoulder blade is a large, thin, relatively flat bone. Together with the clavicle (collar bone) and sternebrae (breast bones), the scapula is at the center of the sloth’s so-called shoulder girdle, playing an essential role in controlling movement of the upper arm as well asi_Beam_drawing_large magnifying the arm’s power.   The smoother, flatter side is the ventral or “in” side, configured to fit snugly against the ribs on the back.  The scapular spine marks the dorsal or “out” side, serving to bring  rigidity to this otherwise amazingly thin bone, like the upright part of an “I” beam.

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Walk Like a Sloth–lesson 9: the clavicles

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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The clavicle or collar bone connects the scapula or shoulder blade to the sternum or breast bone. Along with the scapula and proximal end of the humerus (upper arm bone) the clavicle makes up the sloth’s so-called shoulder girdle and plays an essential role in helping control the sloth’s arm movements. The larger rounder end of the clavicle connects to the manubrium of the sternum while the smaller thinner end connects to the acromion of the scapula. Both ends are rough and pitted, not smooth as on other joints, serving to better anchor important ligaments within the shoulder girdle. These are synovial joints–in life you would find a cartilage pad between the bones and a layer of fluid to lubricate and cushion the shocks these joints must absorb.

Like your clavicle, the sloth’s collar bone is curved slightly to wrap around the front of the chest (bow side out) and attach to the scapula. Your clavicle bends forward slightly again, making an “S” shape, Megalonyx’s doesn’t, but there is great variability in shape among humans as well as sloths. (Aiello and Dean, 1990)  With the medial or “in” side versus the lateral or “out” side determined, we just need to identify the top or superior side from the inferior side or “bottom.” Look for the cone-shaped  conoid tubercle about 1/3 of the way from the thinner (acromial) end. That goes “down.”  It anchors a ligament that attaches the clavicle to the coracoid process of the scapula.  Note the anterior expansion of the clavicle at the lateral end.  That’s extra surface area for attaching part of the deltoid muscle.

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SLOTH CLAVICLES, adult (top), Toddler (bottom)

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Walk Like a Sloth–lesson 8: teeth

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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tooth section

SLOTH CANINIFORM, cross section

This caniniform tooth (“in the form or shape of a canine”) is from the sloth’s lower right jaw or mandible.  The sloth had four (4) caniniforms (sometimes called “tusks”)–two (2) on the bottom and two (2) on top.  Slip the tooth out of its alveolus (socket).  The occlusal surface  (top, biting surface) is smooth and slopes toward the tongue or lingual side and away from the outside or labial side. The root is open because this tooth is ever-growing.  We can’t show you with the plastic prototype, but the tooth is composed of two layers of dentine covered with an outer layer of cementum.  The dentine layers are soft and wear away more quickly than the hard cementum layer as the sloth chews, leaving a hard raised edge for cutting.

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SLOTH CANINIFORM, LOWER RIGHT, ADULT
view from lingual side, root on left

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Walk Like a Sloth–lesson 7: the mandible

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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Only part of the mandible is in the traveling trunk–the chin and right lower jaw of the adult. The large alveolus or cavity on the superior (top) surface holds the front tooth or tusk, called the caniniform (“in the form or shape of a canine,” see tooth lesson).  On the other side of the chin there’s part of the corresponding alveolus for the left caniniform.  The narrow inward sloping shelf of bone anterior and between the front teeth is called the mandibular or predental spout.  The teeth behind the caniniforms are called molariforms (“in the form of a molar”).  A gap or diastema, separates the caniniform from the first molariform.   Notice how deep the alveoli are and how close they approach the bottom of the jaw.  These are long, deep teeth–the term for that is hypsodont–more on that in the tooth lesson too.  The small hole at the side of the chin is the mental foramen which provides an exit for the mental nerve and blood vessels that serve the front teeth, chin and lower lip. There’s another exit on the opposite side of the jaw.  The mandible has (4) teeth on each side—one (1) caniniform and three (3) molariforms.

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MEGALONYX MANDIBLE,
right anterior

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Walk Like a Sloth–lesson 6: sternebrae and sternal ribs

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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SLOTH STERNUM,ventral view (Owen, 1842)

SLOTH STERNUM,ventral view (Owen, 1842)

Megalonyx has a sternum or breast bone composed of a stack of eight (8) separate bones called sternebrae  (singular is sternebra).  The sternebrae connect to the ribs through eight (8) pairs of bones called sternal ribs that curve around the front of the chest and connect to the true ribs with a thin layer of cartilage.  The triangle wide, flat, smooth back of the sternebrae is the “in” side or dorsal surface. Notice the protruding opposite side, called the ventral process, is heavily textured.  As in all Megalonyx bones, this increases the surface area for anchoring the sloth’s powerful muscles, in this case, the pectorals.

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Walk Like a Sloth–lesson 5: the astragalus

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Introduction to Walk Like a Sloth: lessons in ground sloth locomotion

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 The astragalus (called the talus in humans) is basically the fulcrum of a lever rocking on the calcaneum or heel bone like a teeter-totter, moving the foot up and down when muscles pull on the Achilles tendon. valentineThe astragalus has a convex side (the superior or “up” side) and concave (inferior or “down”) side.  With the convex side up you’ll see a heart-shaped articulating surface—orient the bone so it looks like a Valentine’s Day heart.  The smooth surface on the left side of the heart wraps around the bone and makes a right angle down to the bottom.  The fibula articulates on the left (lateral) side of the heart, extending down the outside of the astragalus, providing stability with its projecting tip. The tibia sits on the right (medial) side of the astragalus providing the main support for the leg column.  A pear-shaped cupped depression projects forward and slightly to the right pointing to the sloth’s big toe—that articulates with the navicular bone, which serves as the base for toes 1-3.  Extending laterally (to the left) from the navicular cup is the articulation for the cuboid bone, which connects to toes 4-5.  Turn the bone over.  The astragalus rocks on the calcaneum in a concave saddle formed by two articulating surfaces– a large one posterior and a smaller one anteriorly and medially (on the left side now).  Note that the joint runs diagonally across the bottom– the astragalus rides “sidesaddle” on the calcaneum. leidy calcaneumThe deep groove separating the front and back joints is called the interosseus groove or sulcus tali.  It lines up with a corresponding groove on the calcaneum forming a deep bony canal called the sinus tarsi. The rough texture of the canal serves to anchor a strong inch-wide ligament called the interosseous talocalcaneal ligament (ITCL) which ties these two critical foot bones together.

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