 Today everybody, Dr. Sanjay Sanyal, Professor Department Chair. So this is going to be a static dissection demonstration of the vertical column, the back, the spinal cord, the spinal nerves, the spinal subarachnoid space, and a few points about the posterior costal space. We thought it would be a good idea to give you a quick overview of what it takes to expose the posterior aspect, especially the vertebral canal spinal cord. So this is it ever, and we spent quite a bit of time with the assistance of my students to dissect out the suboxibital region. We have excised the squamous part of the occipital bone, in effect we have done a posterior fossa craniactomy, and we have opened out the posterior cranial fossa. And we can see the two cerebellar hemispheres. Then we continued the incision down, we had to remove the paraspinal muscles here and on this side. Paraspinal muscles being the outermost layer, the rectus spinal muscles, composed of the heliocostalis, lungisimus and the spinalis, and the deep muscles namely the transversal spinalis. We also had to cut the lamina of the vertebral column, all the way from the cervical region to the sacral region. So in other words we had to do a laminatectomy. And then we had to split over the dura. And you can see the upper portion of the dura has been split and what you can see inside is the spinal cord. So this process is an extensive process, it took almost one week. Six students were working almost continuously and it's not something which we can do on a daily basis. Now let's give you a quick overview of the dura erector of the biometer and the subarachnoid space where the CSF is located. And the next slide shows further continuation of the opening of the dura. So what we see essentially here, this is the spinal subarachnoid space. And inside the spinal subarachnoid space we can see this spinal cord. This spinal subarachnoid space is filled with CSF in life. In a cadaver we cannot see the CSF because it has been drained away. And we can see that this is the combined dura and the arachnoid which I should show you more clearly in the next view. So this is an enlarged view of the same thing shown transversely and we can see the cut margin of the dura arachnoid. The inner smooth widest structure is the arachnoid. It is composed of flat mesothelial cells. The outer portion is the dura which is tough, leathery and membranous. So this is the dura border cells are closely adhered to the mesothelial cells constituting what is known as the dura arachnoid interface. This shiny translucent membrane that is covering the surface of the back of the spinal cord, this is actually the biometer. Now it's time to give a quick overview of the spinal arteries especially the posterior spinal artery and how it relates to the posterior spinal artery syndrome. Embedded within the biometer we have these plexus that we can see here. This is the plexus of lascular structures. More precisely on the posterior aspect we have two spinal arteries, posterior spinal arteries, both of them come out from the vertebral artery. So this is one posterior spinal artery that we can see faintly here and this is the other posterior spinal artery which we can see. The posterior spinal arteries they descend down typically just medial to the attachment of the spinal nerve roots which I shall mention just a little while later. These posterior spinal arteries they are supposed to be deficient in the region of T1 to T3. This is actually the thoracic segment of the spinal cord that we are showing right now. And they have to be reinforced by means of the segmental medullary arteries which enter the vertebral canal through the intervertebral foramen from outside. And therefore they form a plexus and that is the plexus that we can see here. So if for some reason in this region we have occlusion of the posterior segmental medullary arteries and the spinal arteries are also weak then we can get ischemia of the T1 to T3 segment of the spinal cord. What is referred to as the posterior spinal artery syndrome. Posterior spinal artery supplies the posterior one third of the spinal cord specifically the dorsal columns and the personal nerve roots. Posteriorly we also have three spinal veins, longitudinal spinal veins and we can faintly see them here. One of them is this one, one of them will be the midline and one of them is this one here. The three posterior spinal veins they ultimately they read through the intervertebral veins and they drain out. They also communicate with the internal vertebral venous plexus which is situated outside the dura but they are not present here. This is the spinal subarachnoid space. Now let's take a look at how the spinal nerves emerge by means of the dorsal rootlets forming the dorsal spinal nerve from the dorsal lateral aspect. Now let's focus on this spinal nerve roots that we can see here. The roots of the spinal nerves they do not arise by a single nerve fiber as we can clearly see they arise by means of multiple branches. These are referred to as the rootlets of the spinal nerves. So if this was let's say for the sake of discussion this was the T3 spinal nerve. Then these are the rootlets of the T3 spinal nerve and this segment of the spinal cord will be referred to as the T3 spinal segment. That's how we count the spinal segments because as such there is no other indication on the back of the spinal cord or the front for that matter which indicates the spinal segments except the attachment of the rootlets. So therefore we can see the rootlets here, we can see the multiple rootlets here, we can see the rootlets here and here and so on and so forth. Now let's follow the course of the spinal nerve from its origin in the spinal cord through the dural sheath to the outside. Now let's come and see how the spinal nerves emerge. When the spinal nerves are emerging they pierce through the dura arachnoid. So therefore the inner surface is the arachnoid as I mentioned or outer surface is the dura. This is the small portion of the spinal nerves which are inside the subarachnoid space and here they are based by CSF. This provides a small area for drainage of CSF by means of the perineural lymphatics. So apart from the usual drainage of CSF these small portions drain through the lymphatics around the spinal nerves. Then these spinal nerves they pierce through the dura arachnoid and in so do we, the dura arachnoid and also the pie as a matter of fact they all merge with the epineurium of the spinal nerves. And we can see that much more clearly in this view here. Since we are in the thoracic region let's see how the emerging spinal nerves become the intercostal nerve. Once the spinal nerves pierce as I said we are in the thoracic region the back of the thoracic region they become known as the intercostal nerves. And we can see one intercostal nerve here we can see another intercostal nerve here. These intercostal nerves initially they run in the endothoracic fascia and we can see they are accompanied by vessel cells. Typically the vein is on the top the arteries below and the nerve is at the lower most. Vane artery nerve they run in the coastal groove on the under surface of the rib above and here they have been displaced down a little bit. So this is the orientation and as they go further they enter into the intercostal muscles between layers 2 and 3. Layer 2 being the internal intercostal muscle and the layer 3 being the innermost intercostal. In this particular region this is the region of the angle of the ribs which was attached by the erector spinae muscle. Therefore in this region the innermost intercostal is called the subcostal muscle. So therefore it runs between the subcostal muscle on the inner surface and the internal intercostal muscle and then they continue. And when they continue they also give the collateral branches. And since we are in the posterior cavity of the chest wall I could not resist telling you a little bit about posterior cavity. Since we are also on the back of the chest I will use this occasion to mention a few quick words about thoracotomy, posterior thoracotomy. When we are doing a posterior thoracotomy we make an incision on the surface of the rib and that incision is made like this on the surface of the rib. And then we split the periosteum above and the periosteum below and then we reflect the periosteum and then we shell out the rib from inside by means of a rib cutter. By so doing we have the added advantage of getting the space above and the space below. So therefore it gives a wider exposure. Subsequently when we close the periosteum, osteogeneic layer of the periosteum allows the rib to grow back. And by doing this procedure we also avoid damage to the intercostal neurovascular structures both above and below. So this is the approach to posterior lateral thoracotomy. So these are the points which I wanted to mention to you about the spinal subarachnoid space. CSF, the spinal nerves, the biometer duroarachnoid and the posterior spinal artery and the vein and the posterior spinal artery syndrome.