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Yellow Ligament vs White Ligament
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David Genadek
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 Posted: Sat Oct 24th, 2009 04:17 pm
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Deb,

In class you made a significant point of discussing the difference between yellow and white ligament. I have been trying to understand the importance of the difference between the paleontologists perspective and the veterinary perspective. I think I get it.

I have been looking at the rhomboids and the Trapezius and how they attach to the supraspinous ligament. My first thought was that we have muscles that are attaching to something other than bone. My train of thought was surrounded around the governing of the spinal limbs as it pertains to the tensioning of the supraspinous ligament and the effect it would have on the front leg. It seemed at first glance that it would be like having an attachment of the muscle to a regular bone and the other side being attached to a rubber bone that could go from rubber to Rod. Then it came into my mind about what you were saying in regard to yellow ligament from the paleontologist perspective as having perhaps once been a muscle. That made me realize that the rhomboids in the thoracic muscles are more like a fanning out of the supraspinous  ligament  rather than merely an attachment to it.

Are systems like this why the distinction is of great importance?
David Genadek

DrDeb
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 Posted: Sat Oct 24th, 2009 09:40 pm
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Dave -- as usual with you -- great question.

The first task here is to identify the nature of the supraspinous ligament, i.e. what in my books I call the 'dorsal ligament'. This is the pair of cords, sometimes fused to each other across the midline, sometimes not, that are about the diameter of a person's index finger and that run parallel to each other along the back just atop the spinal processes. This ligament is, in fact, a complex interdigitation of white (inelastic) fibers and yellow (elastic) fibers. It is continuous with the funicular or cable-like part of the nuchal ligament. Let me quote from an important footnote in Sisson & Grossmann's 'Anatomy of the Domestic Animals':

"No line of demarcation exists between the ligamentum nuchae and the lumbo-dorsal part of the supraspinous ligament, since the change from the elastic to the white fibrous structure is gradual."

And this is exactly what we find upon dissection, except we need to add that there is never actually any change in the composition of the supraspinous ligament behind the top of the withers: in other words, the whole thing from that point to the rear is composed of a mixture of elastic and inelastic fibers. The white, inelastic fibers that compose it are structural and they are there to serve as the dorsal component of the greater population of fibers that are the 'deep intervertebral sheathing' that holds the whole spine together. These are the fibers that you and I were working on after the class had gone home last time....in other words, the last part to be knacked out, the deepest level, the tough stuff you have to cut through in order to get the backbone sectioned up so that it will fit into the maceration barrels.

The elastic fibers that compose the supraspinous ligament, on the other hand, ensure that the spine as a whole from the top of the withers to the dock will respond elastically....that it will have the capacity for 'elastic rebound'.

Now, as to how the thoracic trapezius and rhomboideus are attached. The trapezius, which is the more superficial and slightly the more posterior of the two muscles, originates from an expansion of the supraspinous ligament at the level of thoracic vertebrae nos. 3-10. The rhomboideus arises from the dorso-scapular ligament, which itself is attached from thoracics no. 2 to about 8.

As with other ligamentous structures set forth in anatomy texts, though the names make them sound as if they are separate and different structures, they really aren't.  It is helpful to envision them merely in terms of their spatial geometry. If you were to strip off all the muscle tissue and fan out these sheets of connective tissue, what you would find is that they are all productions from the conjoined periostea of the whole row of dorsal processes. Being part of it, they can well be said to be 'firmly anchored to it'. So you envision a sheet of tissue widening out to either side of the spine, like a sheet that's been clothespinned to a rope. The rope (the supraspinous ligament) runs lengthwise, while the sheet fans out to the side. It's a raggedy sheet, being wide over the hips and loins, narrower under where the saddle goes, and then having several fairly long, straplike flaps at the front.

There is a wide place in the supraspinous ligament (the rope) as it passes over the top of the withers. From this wide place and/or from the conjoint periostea just deep to it, a whole set of sheets arises....as if you had six sheets, laid one on top of each other, clothespinned to the line.

Now, remember that even though periosteum has very little elastic stretch/yellow fibers in it, that the supraspinous ligament is unusual in being mixed. So the sheets that come off to the sides have a very high percentage of elastic/yellow fibers, as does the analogous tissue in the neck, which would be the lamellar part of the nuchal ligament. It's just that the funicular part of the nuchal ligament (the rope) is also very stretchy, while the supraspinous ligament is less stretchy.

So, now, back to the sheets that are over the withers. The most superficial comes directly off the supraspinous ligament, and this sheet is called the 'expansion of the supraspinous ligament'. The thoracic trapezius is attached to this sheet.

The next sheet deeper is the dorso-scapular ligament, which is continuous with -- in other words it is just one of the straplike flaps coming off the front end of -- the highly important lumbo-dorsal fascia that you saw in the dissecting class, that overlies the horse's loins -- one of the most important of all the passive structures. This sheet is fairly thick and quite elastic. Such sheets are sometimes termed 'aponeuroses', and it is from the forward part of this aponeurotic sheet that the thoracic rhomboideus arises.

Below these two sheets are about four other, thinner, elastic leaves, which give origin fore and aft to the cervical rhomboideus, complexus, splenius, the serratus ventralis, parts of the latissimus and longus costarum, etc.

So much for the anatomy and structural geometry, and hopefully this clarifies your picture of how the upper shoulder muscles are arranged (on which the fore arch of the saddle must rest). As to the importance of the distinction between white and yellow ligaments:

-- The distinction is important because HISTOLOGICALLY (i.e. in microscopic thin-section) the nature of white vs. yellow connective fibers is grossly different

-- because FUNCTIONALLY elastic vs. inelastic tissue are grossly different

-- because in terms of INJURY ETIOLOGY elastic vs. inelastic tissue are grossly different, as are their potentials and mode of healing

-- because DEVELOPMENTALLY and in terms of their EVOLUTIONARY HISTORY, the two tissue types are grossly different.

I think it's important for students to distinguish between the two because, since in conventional anatomy books they are all lumped together under the term 'ligament', this is confusing and misleading. As you heard me say in class, the usual definition of 'ligament' (a structure that connects bone to bone) is itself faulty, since most muscles would qualify to be called 'ligaments' by that definition. There has to be clarity on all these points before anything can be usefully discussed or profitably researched.  

Cheers -- Dr. Deb

David Genadek
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 Posted: Wed Oct 28th, 2009 04:43 pm
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Deb,

I'm a little confused about which section of the dorsal ligament has more of the elastic tissue?  This seems an important distinction to me as I am viewing  lamellar portion of the system the same as I would view the spinal processes. Both are connecting the dorsal ligament to the centra. One is flexible one is stiff so does the nuchal portion becomes stiffer to allow for the flexibility of the lamellar sheet? Does the stiffness of the spinal processes allow for the flexibility of the supraspinous section of the ligament or is it stiff to stiff ,flexible to flexible?    
   I can sure see where the definitions are inadequate. The dorsal ligament serves as a ligament  to attach the poll to the croup and everything in between but it is also acting like tenden attaching all the muscle groups to the poll and the croup. I guess the argument is whether to classify by tissue type or by function. Tissue type is nice and clean especially when you're looking at something like this which has multiple functions.
     I have been puzzling over  why if everyone is trying so hard to keep their saddles off the shoulders ,why we have so many screwed up shoulders and whither areas. This makes that all makes sense since those major muscle groups are directly attached to the croup and we set the saddle too far back it prevents the horse from using its hind end correctly and since that is necessary for the proper functioning of the muscles in the shoulder it makes sense. So the dorsal ligament is like a rope with a bunch of bungee cords attached to it if the rope is slack none of the bungee cords will be able to function to their optimum degree? I think that might even be an analogy you use but I didn't get it at the time.
Dave

    

DrDeb
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 Posted: Thu Oct 29th, 2009 07:51 am
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Dave -- The nuchal ligament has the highest percentage of elastic fibers and is therefore the most stretchy. It is obviously yellow to look at.

Where it comes back to join on to the top of the withers, there is a zone of intergradation which is visible to the naked eye and that also shows up if tissue samples are taken (and thin sections made) from successively more posterior points from the front of the withers to the top of the withers; the percentage of inelastic or white fibers increases, and the dorsal ligament becomes whiter in color. From the top of the withers back to the root of the tail, the composition of yellow/elastic vs. white/inelastic remains about the same, so that this section of the supraspinous ligament is less stretchy/more structural than the nuchal ligament, yet still retains some elastic capacity.

The lumbodorsal fascia -- the portion of the sheet that expands outward to cover the loins and the anterior part of the top of the croup -- is yellower/stretchier than the supraspinous ligament itself. The inner 'leaves' anteriorly, to which the deeper muscles like the complexus and latissimus are anchored, are also obviously yellow and stretchy. The more superficial 'leaves' on the anterior end, i.e. the dorsoscapular ligament and the 'wide spot' to which the thoracic trapezius is attached, are whiter and less stretchy, in other words about the same as the supraspinous ligament.

Yes, I often use the clothesline/bungee cord analogy in class. You've seen me do this by using a lead-rope to which I have attached either pieces of cloth (sheets) or else other cords. If weights are attached to the bottom edge of the sheets, or to the bottom ends of the cords that hang down, and if the two people holding the main clothesline/bungee cord pull apart -- against each other like a tug of war -- they pull the main line taut and this causes all the weights to rise; it supports the weights. If the people holding the main line push together, so that the line becomes slack, then of course all the weights sink; they are then not supported.

This is what 'passive support' means. The main line -- meaning the nuchal ligament + the supraspinous ligament -- contains no contractile fibers. It is not a muscle. It cannot contract by itself; it cannot self-shorten. Therefore, if it becomes shorter from end to end -- as it will if it is slacked -- then something else is shortening/slacking it. If it becomes longer from end to end -- as it will if it is stretched or tensioned -- then something else is causing that, also, just as the people holding the main line are the cause of the line's becoming either slackened or tautened. The line itself is passive; muscles are the only body tissue that can be said at any time to be 'active' in the biomechanical sense. And yet, even though the supraspinous and nuchal ligaments are passive, they are able, because of the many connections downward that they make to the bones of the neck, thorax, and lumbar spans, to support the spine in a biomechanically crucial way.

So yes, it is extremely important that the horse always be able to coil its loins, for unless it can do that, it can neither arch the freespan nor raise the base of its neck; and in being deprived of the ability to do these things, the passive support system is disabled. This is why it is important for all ridden horses, at all times, to maintain a degree of collection. Not just reining horses or dressage horses need collection; enduro horses, trail horses, working horses of all kinds, must have it too; they need to be taught this as a postural habit, so that they maintain at least that minimum degree of collection which we call 'rounding up' at all times while bearing weight upon their backs. For unless they do this, the weight of the rider will itself act to push the center of the back down and thus slack and disable the passive support system. Of course, an ill-fitting saddle will add to this effect, as will bad riding technique where the rider sits heavy on the rear, posts heavily, bounces while attempting to sit the trot, or sits heavy on one side all the time: for these things all provoke the horse to contract the muscles that lie along the spine, that parallel the supraspinous ligament; and when those muscles contract bilaterally, that is a strong and effective way to hollow the back.

The time factor also needs to be considered: because the longer a horse goes 'hollow' -- I mean not years, but how many tens or hundreds of steps already -- the more the damage multiplies, until sooner or later there will be actual lesions of various types that the veterinarian can detect and name. In other words, if you let your horse go hollow or even encourage him to go hollow, he'll finally wind up not just moving badly but actually hurting himself. Some of those lesions will show up in the horse's back; more often they will show up in the limbs. The bottom line is if you want your horse to move well, stay sound, and last a long time -- not to mention enjoy his work -- then you need to get a saddle that fits right, learn to ride well, and learn how to cause a horse to round itself up.

Cheers....Dr. Deb

David Genadek
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 Posted: Fri Oct 30th, 2009 04:34 pm
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Deb,

I was intrigued when you said that the two sides of the supraspinouse  ligament are not always connected. It reminded me of the Mustang sacrum I picked up in the Pryor mountains. It was clearly a young horse so I'm wondering if this was just the horse that was not fully developed or if you actually see this forking off in the spinal processes in some animals?
David Genadek

Attachment: pmsacrum.jpg (Downloaded 208 times)

DrDeb
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 Posted: Sat Oct 31st, 2009 07:21 am
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David, yes, this is the sacrum from a young animal. You can know that by observing the tops of the dorsal processes: they are rough in texture and, in side view, they look flat instead of rounded upwards. They have this appearance because, not having completed their development before the animal died, the physis or "end cap" that topped each dorsal process was still, at the time of death, separated from the dorsal process by an interval of cartilage.

The cartilage was the "growth plate" where new bony material would have been laid down which would enable the dorsal process to lengthen upward as the animal grew. When the animal died and its carcass rotted, however, the cartilage -- being soft tissue -- rotted too. Only bony material was left.

Undoubtedly you picked up the sacrum from a spot on the ground where it was lying sort of by itself, separated from other bones of the skeleton which might have been scattered over several yards' distance in any direction. Out West, coyotes scavenging the carcass are the main reason for this; they grab various pieces and carry them off a ways before they lie down to chew on them.

So, the sacrum likely wasn't lying right where the horse died. If it had been, and you had taken the time and trouble to sift the earth within one foot in any direction of the sacrum, you would have found a number of little, round, domed bones about the size of the end of your thumb. These would be the "end caps"which were in fact present in the young horse, situated above the growth cartilages and forming the ossification centers for the tops of the dorsal spines. What you show us in the photo is the body of the sacrum missing all of these physes.

Now, as to the left and right cords of the supraspinous ligament not always being connected: this has nothing at all to do with the sacrum you show us. When I said that, I did not mean "sometimes connected" in the temporal sense, but rather in the spatial sense. In the living horse, the left and right supraspinous cords are united over the top of the withers; this occurs in the wider, thickened portion to which the thoracic trapezius is attached. The two cords are normally separate from about the base of the withers back to the peak of croup. Behind that point, they become smaller in diameter, in some individuals remaining separate, while in others we can find more or less numerous connections that bridge the midline.

So the separateness of the cords that form the left and right supraspinous ligaments is not related to the doubleness of the dorsal processes on the sacrum you present. What these are is a fairly rare abnormality which is of the same nature as spina bifida but of mild degree. You notice that the vertebral centra are not involved; if they had been, the horse would likely not have been able to stand at birth.

I also want to add just as a cautionary note -- though your photos are pretty good, the specimen is not complete. Would you be willing to photograph the sacrum from the front end (the wide end, the end away from the doubled spines) -- because I want to make double certain that the bone is in fact from a horse and not from a cow. The front view is diagnostic. Doubled dorsal spines are much more common in cattle than horses.


Great specimen, though, certainly one to add to the collection. Cheers -- Dr. Deb

 

Last edited on Sat Oct 31st, 2009 06:53 pm by DrDeb

David Genadek
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 Posted: Sun Nov 1st, 2009 09:03 pm
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Deb,
     The sacrum was found in the Pryor mountains alng with the other two I am going to post. We were told that it unusual to find bones but I found several big box's worth of bones. There seems to be a general location where they seem to go to die. It is as though they know if they die in this area that the contours of the land  are such that the remains will be carried away by the floods in the spring.
  I reshot that sacrum along with two others .
David Genadek

Attachment: mustang-sacrum1.jpg (Downloaded 141 times)

David Genadek
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 Posted: Sun Nov 1st, 2009 09:06 pm
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This one is interesting in that it has more curvature crainally to caudaly.  Interesting to ponder if that would have a compounding effect on the tensioning of the supraspinouse ligament.

Attachment: mustang-sacrum2.jpg (Downloaded 143 times)

David Genadek
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 Posted: Sun Nov 1st, 2009 09:11 pm
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On this one the asymetry of the tops of the spinal process strikes me and has me wondering if such asymmetries could cause handedness or if handedness could cause the asymmetries.

Attachment: mustang-sacrum3.jpg (Downloaded 143 times)

DrDeb
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 Posted: Sun Nov 1st, 2009 10:34 pm
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Hi, Dave, thanks for the re-shoot. Yes, they are all horses. The first one is broken so that it is missing the anterior segment, but the anterior view is still diagnostic horse vs. cow.

The handedness causes the asymmetries, not the other way around. The skeletal system is among the last to form in the embryo....the kidneys come first, then the nervous system, then circulatory, long before any bone ossifies. "Eye dominance" is a very good explanation for the root-cause of side preference in horses, both because it relates to the nervous system and because of the fact that the horse has its eyes on the sides rather than the front of its head.

The curvature of the 2nd specimen does not more highly tension the dorsal ligament system. What it did do was produce more of an "apple butt" on that animal.

What tensions the dorsal ligament system is effort made by muscles that are attached below the spinal chain. Whether they can effect a higher degree of tension in one animal vs. another will be a product not only of how strong those muscles are, but also of how large an angle they can move the pelvis and sacrum through when they coil the loins. If you think about this, you will realize that a more mobile lumbo-sacral joint would be one factor that could contribute, but so would having the sacrum shaped as a flat line or even curved upwards, creating a configuration like a compound bow. I have never, in fact, seen a horse sacrum that was curved upwards; but there are many straight ones. A longer sacrum would also increase the power of loin coiling to tension the dorsal ligament system.

It is very interesting indeed how much skeletons vary from one to another individual within the same species. I'm tickled you picked up boxes of bones....they are quite valuable, as they supply information that may be in contrast with what we can observe in the domestic horse population. Did you happen to get any skulls or jaws? I'd like to see images of the teeth. Cheers -- Dr. Deb

David Genadek
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 Posted: Sun Nov 1st, 2009 11:04 pm
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No jaws no skulls I figure the coyotes probaly drag the heads off to suck brains out. I think if I followed the canyon down I would find a lot more stuff but I didn't have time for that adventure this trip. I did get a whole pelvis, a bunch of scapulas, nearly a complete lumbar span, lots of vertebra, leg bones (one had a bullet in it)and  ribs, all and all quite  a mish mash. The force of water in this area could wash away just about anything. Some of the better stuff I found was hung up on rocks.
David Genadek

Seglawy Jedran
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 Posted: Mon Nov 2nd, 2009 04:28 am
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Dear Dave:  Suck the brains out? Why? Aside from the general if its meat eat it world view of coyotes in their  oh so irreplaceable recyclers-scavengers niche. Do brains have some type of rare nutrient value?
best wishes
Bruce Peek

DrDeb
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 Posted: Mon Nov 2nd, 2009 06:22 am
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Yes, Bruce, brains  (called 'sweetmeats' or 'sweetbread' in cookbooks) are full of nutritious and high-calorie fat.

Look in Kay Berhensmeyer's textbook of taphonomy, which is the science of tracking how carcasses are scavenged, degrade, and then become buried, permineralized, or fossilized: the brains are one of the first items that a scavenger species will go after, selecting that part of the carcass along with the liver as being the most nutritious.

It wouldn't be quite correct or accurate to say that coyote's "suck" the brains out, but I think Dave was just indulging in hyperbole there. They do what domestic dogs do: crunch into the cranium, breaking it open; then lick out the brains, which are relatively soft even when fresh.

Great discussion for Hallowe'en, guys....the worms crawl in, the worms crawl out....I'm sure you know the rest of the words! Cheers -- Dr. Deb

David Genadek
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 Posted: Mon Nov 2nd, 2009 04:54 pm
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I lied I do have a few shots of teeth. I took these in 2007 and they are of a horse
named Plenty Coupes who was killed by lightning in 2001.

Attachment: plenty-coup-lower-jaw.jpg (Downloaded 123 times)

Last edited on Mon Nov 2nd, 2009 04:59 pm by David Genadek

David Genadek
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 Posted: Mon Nov 2nd, 2009 04:56 pm
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k

Attachment: plenty-coupes-front-teeth.jpg (Downloaded 119 times)


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