 This is going to be a demonstration of the intrinsic back muscles, the true back muscles. This is the prone cadaver, we are demonstrating the right side. Just to bring you up to speed, we have slit open and reflected the posterior leaf of the thoracolumbar fascia and you can see the leaf and this is the medial half of the leaf. So we have split it and we have reflected it and on so doing we have exposed the erector spinae muscle. You can see this strong thick tendon lifted up by my assistant here. This is the tendon and the fleshy muscle which we can feel on either side of the midline in our backs when we feel them. And this is the true erector spinae muscle which is responsible for maintaining the erect posture in the Homo erectus. That's why this group of muscles are referred to as the erector spinae muscle or the sacro spinalis. This strong composite tendon arises from the sacrum and from the aliac bone. That's why this is also called the sacro spinalis. This group of muscle actually has got multiple levels but here we will demonstrate the first level. The first level is subdivided further into three broad groups. The first one, the most lateral is this one here. This muscle going all the way from here and going up and continuing here. This is the ilio costalis. Why is it called ilio costalis? Because it starts from the iliac bone. And we can see multiple slips getting attached to the ribs. We can see one slip going attaching to this rib here. We can see another slip attaching to this rib here. Another slip here. It's going up and we can see multiple slips getting attached to the ribs. That's why this whole portion is referred to as the ilio costalis. And depending on which location it is divided into ilio costalis numborum, ilio costalis thoracis, ilio costalis ceresis. So this is the lateral most part. Next just mediate to the ilio costalis we have this muscle here. Before I proceed with that I need to tell you something that there is a lot of interchange of fibres. These muscles are not discreetly independent of each other. And we can see that there is a lot of interchange of fibres between them. The next group of fibres is the longascus. This also rises from the same common tendon from the iliac bone and the sacrum. And it goes all the way up and it goes right up to the occipital bone which we cannot see here. But if you put my finger you can feel it. That is why it is referred to as the longascus. And again this muscle we can see that apart from exchanging fibres with the ilio costalis it also has got different segments which are arbitrarily named as ilio costalis thoracis, ilio costalis, ceresis and capitis. So this is the next muscle. And not only that we can see that it is also interchanging fibres with the next group of muscle which I am going to tell you just now. The medial most group of muscle is this one. This is the smallest group here and I am tracing my finger along it. This is the one which is right next to the spinous processes. This is the spinalis muscle. So I have lifted it up here now. This is the spinalis. This is the smallest group and it is right next to the spinalis processes and again depending on the location it is referred to as the spinalis thoracis, spinalis services. So therefore we have ilio costalis, longascus and spinalis. These muscles they are referred to as postural muscles. They are type 1 red muscles, slow twitch, not easily fatigable and they are the ones which are possible for maintaining the posture and this is because they are in a constant state of contraction or tonic state of contraction by higher cortical control by means of the vestibulospinal and the reticulospinal tracts. That is why they are referred to as postural muscles. If these muscles are completely removed from here then deep inside we will see the next group which we cannot see very clearly here which will be located between the spinous processes and the transverse processes and a little few fibers of those are visible here when I retract them. They are referred to as the transversal spinalis which are not so important in maintaining the posture but they are also true intrinsic back muscles and they are divided into again three subgroups from superficial to deep. They are called semi-spinalis, multi-fitters and rotators and there are some deeper group they are called the levator, costorum, inter-transverse and inter-spinal. So these are the complete group of intrinsic back muscles. So that brings me to the final point. What are the characteristics of an intrinsic or a true back muscle? Number one, they are all enclosed in the posterior and the middle layer of thoracolumbar fascia. Two, they act directly on the vertebral column. Three, they are supplied by the dorsal ramia of spinal nerves. So these are the three important characteristics of the true or the intrinsic back muscles. So that is all for now. Thank you very much for watching. If you have any questions or comments please put them in the comment section below. Dr. Sanjay Sanyal signing out, have a nice day.