Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the muscles of the ventral trunk. So, what we are going to be doing here on this tutorial is looking at this image and describing the different muscles that define the ventral trunk… so, the anterior part or front part of your trunk. So, basically, we are going to be describing the different thoracic muscles and also some of the abdominal muscles that define the anterior portion of the thorax and also the abdomen.
So, as you would probably expect, right now, we’re looking at the anterior view of the thorax and abdomen so the anterior view of the trunk where we can see the different muscles that we’re going to be talking about on this tutorial. But, before I do so, I would like to list the different muscles of the ventral trunk and these include the abdominal muscles, the intercostal and subcostal muscles, the transversus thoracis, the serratus anterior, the pectoralis major and minor, the subclavius, and the cremaster.
We’re going to start off by talking about the first list of muscles which are the abdominal muscles which you can see now on the image on the right side and the list includes the rectus abdominis muscle, the pyramidalis muscle, the external oblique, the internal oblique, the transversus abdominis muscle. So, these are the abdominal muscles that define the ventral trunk.
We’re going to start off with this one that you see here highlighted in green which is known as the rectus abdominis. The muscle is passing through what is known as the rectus sheath which is a tendinous muscle envelope built by the aponeurosis of the lateral abdominal muscles. This muscle has between 3-4 horizontal tendinous intersections which you can clearly see here on this image, and they are adhered to the anterior layer of the rectus sheath giving it its multi-bellied appearance. And, thanks to this muscle, and when you work hard on it, this is when you get then the well-known washboard abs or the well-known 6-pack.
Now, we’re going to be talking about the different origin points for this muscle. It originates from the 5th to 7th costal cartilages and also the xiphoid process of the sternum, as you can see here on this image. So, these are the origin points for the rectus abdominis. It goes all the way to then insert at the pubic bone, to be more precise between the pubic tubercle and the symphysis, which you can also see here on the image. And as a very good muscle out there, we’ll have to talk about the innervation of the rectus abdominis, and the innervation will come from the intercostal nerves… to be more specific, the lower intercostal nerves.
Now, in terms of the different functions associated to the rectus abdominis, one of the main things is that this muscle is able to tense the anterior abdominal wall. As I mentioned before, the rectus abdominis muscle is responsible for the 6-pack abs because of its tendinous intersections which gives the muscle the multi-belly appearance. When the muscle contracts, it’s also able to perform flexion of the lumbar spine. It is also an important muscle for respiration as it helps perform a strong exhalation and creates then intra-abdominal pressure during defecation and/or childbirth. The rectus abdominis muscle is an antagonist to the intrinsic back musculature.
Next muscle that we’re going to be talking about is a tiny, tiny muscle known as the pyramidalis which is poorly developed in humans. Now, the origin, the muscle will come from the pubic bone as you can see here on the image and then will be running within the rectus sheath to the linea alba where it’s going to be inserting, and this is the linea alba as you can see here on the image. As for the innervation of the pyramidalis, the muscle is going to be innervated by this nerve or these nerves seen here highlighted in green, the subcostal nerves, also known as the 12th intercostal nerves. When it comes to the different functions associated to the pyramidalis, while this muscle almost has no functions we’re talking about, but it’s important to mention that it’s able to tense the linea alba in this section.
We’re going to move on and talk about the next muscle in our list. This one that you see here highlighted in green which is known as the external oblique. This muscle is going to be originating from the 5th to 12th ribs and at its origin point, it is highly connected with other 2 muscles, the serratus anterior and the latissimus dorsi. The muscle runs caudomedially to then insert on the half of the iliac crest, the pubic tubercle, the linea alba, and also the inguinal ligament will serve as an insertion point for the external oblique. Now, the innervation of the external oblique is going to come from the intercostal nerves, the lower intercostal nerves, to be more specifically between T7 to T12.
We’re going to move on and talk about the different functions associated to the external oblique. When the muscle produces a unilateral contraction, it’s going to be able to contralaterally rotate the torso while a bilateral contraction will be able to flex the lumbar spine. This happens together with the other abdominal muscles. It also supports forced exhalation and increases the intra-abdominal pressure.
If there is an external oblique, there should be the next muscles that we’re going to be talking about, the internal oblique. Now, the internal oblique is going to be originating from the thoracolumbar fascia, the iliac crest, and also the inguinal ligament. The muscle will be then inserting cranially at the lower costal cartilages and ventrally at the linea alba. So, first insertion, linea alba which you can also see here on this image. And as I mentioned, it’s going to be inserting on the lower borders of ribs 10 to 12. In men, the caudal fibers will be extending to the spermatic cord merging to form another muscle that we’re going to talk about, the cremaster.
Now, we’re going to talk about the innervation of the internal oblique. Now, there is a list of nerves that you need to know. The intercostal nerves T7 to T12 are going to be responsible for the innervation of this muscle as well as the iliohypogastric nerve and the ilioinguinal nerve which are branches of the lumbar plexus.
Next in line will be then the functions of the internal oblique. And, unilateral contraction will be producing ipsilateral rotation and ipsilateral flexion. Now, bilateral contraction will be producing flexion of the lumbar spine together with the other abdominal muscles, and the internal oblique also supports forced exhalation and increases the intra-abdominal pressure.
Next muscle that we’re going to be talking about that you see here highlighted in green is known as the transversus abdominis. This muscle arises from the inner surface of the lower costal cartilages no. 7 to 12. Also the thoracolumbar fascia will serve as an origin point as well as the iliac crest and the lateral 3rd of the inguinal ligament.
Now, when it comes to the insertion point and, as the name suggests, the fibers are running horizontally to the linea alba as you can see here which is now removed but you can see how the muscle is running or the fibers of the muscle are running towards the linea alba with the aponeurosis of the internal oblique. Now, the muscle also inserts at the pubic crest and the pecten pubis. Also, caudal fibers of the transversus abdominis are involved in the formation of another muscle, the cremaster.
And as we’ve seen on the internal oblique muscle, the innervation of the transversus abdominis is going to come from the intercostal nerves T7 to T12 as well as branches of the lumbar plexus, the iliohypogastric nerve and the ilioinguinal nerve. Keep in mind these 2 images. The first one on the left is showing the different intercostal nerves while the image on the right side is showing the intercostal nerve no. 12 which is also known as subcostal nerve.
Now, it is time for us to talk about the different functions associated to this muscle. Now, a unilateral contraction will be causing ipsilateral rotation and ipsilateral flexion. Bilateral contraction will be causing flexion of the lumbar spine together with the other abdominal muscles.
We’re going to go a bit further up to talk about the different muscles that define the anterior portion of the thorax, as you can see here on this image.
We’re going to start off with this one that you see here or these that you see highlighted in green which are known as the external intercostal muscles. They originate from the lower border of the rib and then run ventrocaudally to the upper border of the rib below. Ventrally, these muscles are bordered by the external thoracic fascia. The innervation of the external intercostal muscles come from these nerves that you see here highlighted in green which are the intercostal nerves which arise from the thoracic nerves T1 to T11.
The next group of muscles that we’re going to be talking about that you see here highlighted in green are known as the internal intercostal muscles. As for their origin points, these muscles originate from the upper border of the rib and then they go to insert ventrocranially at the lower border of the rib above. The internal intercostal muscles are going to be innervated by the intercostal nerves, same thing that we saw previously, which arise from the thoracic nerves T1 to T11.
As for the different functions associated to these muscles, the task of the intercostal muscles is to then tense the intercostal spaces during deep inspiration and they also support the breathing mechanism. Keep in mind that the contraction of the external intercostal muscles will cause elevation of the ribs whereas the internal intercostal muscles will be lowering the ribs. So, they are also known as expiratory breathing muscles.
Moving on to the next muscle that you see, or next set of muscles that you see here highlighted in green, the transversus thoracis. This muscle is a thin muscle on the inner surface of the anterior thoracic wall. As for its origin points, it lies on the dorsal surface of the xiphoid process which you can also see here on the image, the xiphoid process, and the body of the sternum as you can see here is going to serve as an origin point for this muscle as well.
Now, from there, there are 5 insertion tendons which run craniolaterally to the cartilages of ribs 2 to 6 giving it a serrated appearance as you can see on the image. Now, the exact number of tendons and the places of insertion vary greatly making the transversus thoracis one of the most variable muscles in your body. Caudally, its fibers course almost parallel to those of the transversus abdominis which is where it got its name from. In rare cases, these 2 muscles can even be found merged together.
Now, the innervation of the transversus thoracis will come from these nerves that you see here highlighted in green. You probably have guessed, you’ve seen several times throughout this tutorial, these are the intercostal nerves specifically 2 to 6. So, the intercostal nerves arising from the thoracic nerves T2 to T6.
Now, the next topic will be the functions associated to the transversus thoracis. The contraction of this muscle will be pulling the ribs’ cartilages caudally and that way supporting expiration, so, when you breathe out. However, it is not considered to be among the primary respiratory muscles such as the intercostal muscles or even the diaphragm because many people have a poorly developed transversus thoracis and show no breathing difficulties. For those reasons, it is only regarded as an accessory muscle of expiration.
Next muscles in line that we’re going to see here also a collection of muscles known as the subcostal muscles. As you can see here, they’re also connecting the ribs, and they will be originating from the inner surface of one rib and then they are inserted into the inner surface of the 2nd or 3rd rib below near its angle, to be more specific, the angle of the rib.
Now, the innervation of the subcostal muscles is carried out by the lower intercostal nerves. As for the different functions associated to these muscles, the subcostal muscles have a similar function to that of the internal intercostal muscles and they depress the ribs which makes them accessory muscles of expiration.
Moving onto the next muscle on our list that you see here highlighted in green, one on each side, is known as the serratus anterior. The serratus anterior is a fan-shaped muscle at the lateral wall of your thorax. It is originating from the ribs specifically rib 1 to 9. And as for the insertion point, the muscle will be inserting at the ventral surface of the medial border of the scapula and, due to its course as you can see here, the muscle has a serrated appearance.
Now, the serratus anterior can be divided into different parts. The superior part which goes from the 1st to 2nd ribs and then all the way to the superior angle of the scapula. There is also an intermediate part which runs from the 2nd to the 3rd ribs and will go all the way to the medial border of the scapula. Another part which is known as the inferior part which runs from the 4th to 9th ribs and then goes to insert on the medial border and also the inferior angle of the scapula.
Now, keep in mind that on the previous slide, I was mentioning that the insertion point for this muscle is the medial border of the scapula. I also mentioned here the superior angle of the scapula and the inferior angle of the scapula as insertion points for the superior and inferior parts which is important to highlight here. So, you can also mention for more detail that the superior and inferior angles of the scapula serve as insertion points for the superior and inferior parts respectively.
Moving onto the innervation of the serratus anterior, the muscle is going to be innervated by this nerve that you see here highlighted in green, the long thoracic nerve. What you see here on this image, this is an anterior view of the shoulder where you see then the humerus, the scapula and the long thoracic nerve, which as you can also see here is a branch of this plexus here, the very famous brachial plexus.
We’re going to move on and talk about the different functions associated to the serratus anterior. Now, the contraction of the entire serratus anterior leads to ventrolateral movement of the scapula along the ribs. Due to this pull, the inferior part at the lower scapula, the shoulder joint is then shifted superiorly. And this shift, will now enable you to lift your arm up to 90 degrees which is then elevation. In contrast, the superior part will be depressing the scapula and does act antagonistically. Another function of this muscle is to then stabilize the scapula within the shoulder. And finally, with a fixed scapula, the muscle lifts the ribs, is able to act as an accessory muscle of inspiration.
We’re going to move on and talk about one of the major muscles that defines your chest. This is the pectoralis major, definitely shapes the surface anatomy of the chest and participates in forming the anterior wall of the axilla. It has a broad origin point so it goes from the medial part of the clavicle; also, the sternum serves as an origin point, and this is a bit of variable information but most sources indicate the costal cartilages 2nd to 6th. The anterior layer of the rectus sheath will be serving as an origin point as well for the pectoralis major.
When it comes to the insertion point for the pectoralis major, this is an easier one. The muscle is going to then or all of the fibers of this muscle will insert together at the crest of the greater tubercle as you can see here. So this is the greater tubercle of the humerus and the crest is found just below it and will be serving as an insertion point for the pectoralis major. The innervation of the pectoralis major will be provided by 2 nerves; the medial pectoral nerve which you see here highlighted in green and the lateral pectoral nerves.
Moving onto the different functions associated to the pectoralis major, this is the most important muscle for adduction and anteversion of the shoulder joint which you see here indicated by this arrow. Adduction when you bring your arm closer to your torso, and also anteversion when you extend your arm forward. This also involved in internal rotation which you see here indicated by this arrow. So if the arms are fixed, the muscle is also able to lift the trunk which can be helpful in climbing or during inspiration.
The next muscle is the smaller one where the very similar name, the pectoralis minor. The pectoralis minor is located under the pectoralis major and both form the anterior wall of the axilla. The contracted muscle can be easily felt or palpated there. The pectoralis minor is going to be originating from 3 ribs specifically; so, from rib 3 all the way to the 5th rib. Then it inserts on the coracoid process of the scapula as you can see here, this projection, of the scapula. And as its big brother, the pectoralis minor is going to be innervated by 2 nerves, the medial pectoral nerve which you see highlighted in green and the lateral pectoral nerves too.
As for the different functions associated to the pectoralis minor, this muscle is able to pull the scapula anteriorly and inferiorly towards the ribs to what we call then depression of the scapula. This movement is both helpful when retracting the elevated arm as well as moving the arm posteriorly behind the back. It’s also involved in elevation of the 3rd to 5th ribs given a fixed scapula then expands the rib cage. By those means, it can also serve as an accessory muscle during respiration.
Next muscle that we’re going to talk about a very small muscle that you see here highlighted in green is known as the subclavius muscle. It’s also known as a muscle, a short muscle of the shoulder girdle. It’s going to be originating, as you can see on the image, from the first rib. So this is the first rib and will serve as an origin for the muscle, just on the border between the bone and the cartilage. As for the insertion point of the subclavius, it inserts at the lower surface of the lateral clavicle, as you can see here, this is the clavicle, thereby, it lies behind the pectoralis major. The brachial plexus and both the subclavian artery and vein pass under the subclavius muscle and, due to its location and relatively small size, it is really hard to palpate this muscle.
As for the innervation associated to the subclavius muscle, we’re looking now at the brachial plexus here and you see this nerve here highlighted in green which is named after the muscle, the subclavian nerve, which is then a branch of the brachial plexus. And just for clarification on this image, you’re seeing the brachial plexus as well as the cervical plexus.
We’re going to talk about the different functions associated to the subclavius. The main task of the subclavius is the active stabilization of the clavicle and the sternoclavicular joint during movements of the shoulder and arm, and in addition its contraction leads to depression of the clavicle and elevation of the 1st rib respectively. And these movements, however, play a rather subordinate role.
Last muscle on our list that you see here highlighted in green, this is known as the cremaster. The cremaster is formed by fibers from the abdominal musculature and covers the spermatic cord and testes. Now, the origin point can be known as the inguinal ligament. And as for the insertion point of this muscle, this is a bit more complicated but it forms a thin network of muscle fascicles around the spermatic cord and testes or around the distal portion of the round ligament of uterus in women.
When it comes to the innervation of the cremaster muscle, this muscle is innervated by the genital branch of the genitofemoral nerve. Now when it comes to the different functions associated to the cremaster, contraction of this muscle is initiated by several stimuli. It lifts the testicles towards the torso known as the cremaster reflex. This mechanism adjusts the temperature of the testicles for an adequate sperm production, one point that the cremaster is not well-developed in females.
