 Good day everybody. Before I proceed with the main dissection, let me give you a quick overview by showing you a pan view of the entire length of the spinal cord. The spinal cord extends just from below the foremen magnum to the lower border of L1 or between L1 and L2. The spinal cord is 45 cm in length. The main dissection will follow just after this. And you can also see that the patient to the cadaver is prone and you can also see the dual sac has been opened out already and the spinal nerves are emerging. If you look closely at the upper end you will see a little bit of the left vertebral artery which I shall describe to you in the subsequent video. Now the main dissection starts. This time we will demonstrate the dual sac and the spinal cord in situ. My name is Dr. Sanjay Sanyal. I am the professor and department chair and the camera persons are Selvi Krishna and Doug McLaren. Okay, so what do you see in front of you? Our boys and girls have done a wonderful job of sawing through the bones to remove all the bones in the laminatectomy to show the spinal cord and the dual sac. So this is the upper limit of the spinal cord and this is the lower limit of the spinal cord. So the spinal cord extends from just below the foremen magna to the lower border of L1 or let's say roughly between L1 and L2. So where my fingers are located that is the extent of the spinal cord. But right now we cannot see the spinal cord because it is covered by the dual sac which I have lifted up here. The dual sac as you can see is brown in color. Before I proceed any further about the dual sac and the spinal cord let me tell you about this particular space where the spinal cord and the dual sac is located. This is the extra-dueral space. What my students have done is they have done laminatectomy of all the vertebrae and they removed the vertebrae to show the extra-dueral space for the spinal epidural space. How does this spinal epidural space differ from the cranial epidural space? There is a fundamental difference. The spinal canal, the spinal epidural space is a true space. It contains fibrofied tissue. It contains a plexus of veins which is known as the internal vertebral venous plexus. It is also referred to as the valvularis veins of bateson. And these internal vertebral venous plexus or the valvularis veins of bateson, they are the main sources of communication between the pelvic and abdominal structures and the cranial cavity. And they are the source of spread of cancer. They are the root of spread of cancer from the pelvis. Like for example cancer of the prostate, cancer of the bladder, cancer of the cervix of the uterus. They can spread to the cranial cavity by means of this extra-dueral space. In contrast, in the cranial cavity, the extra-dueral space is a potential space. That means it becomes a true space only in pathological conditions like for example extra-dueral hematoma. The second fundamental difference is that in the spinal canal, the dura is a single layer of tough membrane structure as you can see here, brown. But in the cranial cavity, the dura has got two layers. An outer layer which is lining the inside of the cranial cavity, the bone, and that is the periosteal or the endosteal layer of the dura. And the second layer is this one which you see here. This is the true layer, the meningeal layer of the dura. So therefore in the cranial cavity, there are two layers of the dura, an endosteal or a periosteal layer which lines the inside of the cranial cavity which incidentally becomes continuous with the periosteum on the outside, and the other is the true or the meningeal layer of the dura. And you can see they have opened the forem and magnum, and my finger is going through the forem and magnum, and it is entering inside. From here you can see it's entering here. So that is about the differences between the spinal epidural space, the spinal dura, and the cranial epidural space and the cranial dura. Now let's take a look at the extent of the spinal cord which I mentioned in the beginning. The spinal cord, the medulla oblongata passes through the forem and magnum. Therefore the spinal cord starts just below the forem and magnum, and it ends. If you were to take a look to determine where it ends, of course I can feel it, but you can't feel it. If you look this is the twelfth rib. So therefore this is the twelfth vertebra, the location of the twelfth thoracic vertebra. Just below that is the location of the L1. Spinal cord ends at the lower border of L1, so roughly between L1 and L2, and I can feel it ending here. So this is the extent of the spinal cord. Below that, again we can see the dural sac. But from L2 to S2, this portion of the dural sac is referred to as the lumbar system. Why is this significant, this lumbar system? I'm going to open it a little later and I'm going to show you. The lumbar system does not contain the spinal cord. Instead it contains only the cordae equina and the phylum terminale. And this place is filled with CSF. And therefore this lumbar system, which extends from L2 to S2, is the site which is used for doing a lumbar puncture and CSF tap. Phylum terminale, which I mentioned earlier, there are two portions of the phylum terminale. The spinal cord, where it ends, that ending of the spinal cord is in the form of a cone and that is known as a cone as medullaris. There the phylum terminale is a little bit of pial fibro-stich neuroglial tissue. It does not contain any nervous tissue. It extends all the way down till the end of the dural sac. That portion of the phylum terminale is referred to as the internal phylum terminale. I'm going to open it and I'm going to show it to you. It is also referred to as the pial part of phylum terminale because it gets a sheath of piometer from the spinal cord. To continue, this phylum terminale then pierces through the dural sac and it goes through the sacral canal and it gets attached to the coccyx. The sacral canal has been opened up here and the coccyx is below. That portion of the phylum terminale is known as the external phylum terminale or the dural part of phylum terminale. Why is it called the dural part? Because as it pierces through the dural sac, it gets a sheath of dura and it gets attached to the coccyx. So therefore there are two parts of the phylum terminale. The next thing that we notice here is the spinal nerves which are emerging. Now you can see some of the spinal nerves because the boys and girls have done a wonderful job of dissecting this out. You can see that one spinal nerve is emerging here. This is the postage intercostal nerve. You can see some spinal nerves are emerging and you can see various spinal nerves are also emerging in the sacral region. These are actually the spinal nerves which are coming out from the phylum terminale and you can see some of the sacral nerves emerging. What do we notice? We notice that this is the region of the intervertebral foremen. This is the region of the intervertebral foremen where my probe has gone in. And as the spinal nerve emerges, it takes a sheath of dura till the intervertebral foremen and then the sheath of dura gets merged with the periosteum of the bone there and then the spinal nerve continues. So the spinal nerves take a sheath of dura and therefore that portion of the spinal nerves are also covered by CSF. And the same applies to all the spinal nerves, even the spinal nerves which are emerging from the cauda equina. That brings me to the next point. What we see here, what I'm picking up in front is not just the dura alone. What we are seeing is the dura. Closely adhering to the inner surface of that is the arachnoid layer. In fact, the dura and the arachnoid are closely adhering at what is known as the dura-arachnoid interface. There is hardly any space between the two. So when we say the dural sheath, we actually mean the dura-arachnoid together. And when I talked about the lumbar cistern, I meant the dura-arachnoid, the space under the arachnoid. And that is the space which is called the subarachnoid space and that is the one which is filled with CSF. The pyrometer, which is the innermost layer of the meninges, closely invests the spinal cord and the brain and it is inseparable from the spinal cord and the brain. So therefore basically what we have is the dura-arachnoid interface, one layer, the pyrometer, another layer and in between is the layer which is the subarachnoid space which is filled with CSF. So that is what I wanted to tell you about this. Having mentioned that, before I open the dural sac, I just wanted to show you something which we have already done. We can see the sciatic nerve coming out here. The sciatic nerve has been dissected out because it is the largest nerve in the human body and it has been traced from the posterior aspect of the gluteal region. It is running between the ischial tuberosity and the greater trope enter. In fact, that is one of the important landmarks of the sciatic nerve and on the right side, this is the left side of the patient by the way, the patient is prone. The sciatic nerve is going down. Normally the sciatic nerve somewhere in the middle of the, or the lower one third of the back of the thigh, it is supposed to divide into the tibial and the common fibular nerve but in this particular specimen, this was incidentally the same specimen which I had done in the earlier decision of the propritial fossa. In this particular specimen, the sciatic nerve divides low down. So instead of dividing at the back of the thigh, the sciatic nerve divides in the propritial fossa into a tibial nerve and a common fibular nerve. And just to give you a quick overview, this is the gastrofemius and the tri... Solius, reflect it and we can see the tibial nerve is continued on the back of the leg. And the common fibular is running laterally. Common fibular runs laterally under the biceps femoris. So that much about the sciatic nerve. So let's come back to where we had started. In the next part of the video, I'm going to demonstrate the actual opening of the dural sac and show you the contents of the dural sac, especially the spinal cord, the nerve rootlets.