The horse needs power to help it gallop quickly and carry a rider or pull a heavy load. The deep muscles help to provide that power, particularly in the hindquarters. The deep muscles lie beneath the superficial muscles, supporting them to do the work. This is the second blog post on equine muscle anatomy - the first covered the superficial muscles on a horse. Do check it out!
Muscle anatomy from the poll of the horse, along the back
The semispinalis capitis originates on the cervical vertebrae and the firs 6-7 thoracic vertebrae and inserts on the occipital bone. It operates alongside the splenius to help lower the neck and bend the neck and head laterally.
The longissimus dorsi include a number of shorter muscles that run along the horse’s back. These have a variety of names, depending on where they attach: longissimus lumborum, longissimus thoracis, longissimus cervicis, longissimus capitis and longissimus atlantis.
The rhomboideus begins at the nuchal ligament and supraspinous ligaments and inserts on the scapula under the trapezius. The muscle lifts the shoulder and the forehand, pulling the scapula forward.
Cervical and thoracic spinalis - integrates with the longissimus dorsi and inserts on the spinal processes of lumbar, thoracic and cervical vertebrae.
The iliocostalis is a very deep muscle that lies laterally to the longissimus, stretching the full length of the spine. Parts of it start in the lumbar area and other segments continue to the third rib. The muscle helps in expiration and extension of the spine.
Transversus abdominus originates on the transverse processes of the lumbar vertebrae and inserts in the xiphoid cartilage and the linea alba. It compresses the abdomen and stabilises the lower spine.
Iliacus originates on the side of the ilium and is inserted into the trochanter minor of the femur. It lies under the gluteus muscle and enables the hip to flex and the femur to rotate out.
The middle gluteus muscle originates from the ilium, from the longissimus dorsi muscle, the gluteal fascia and from the dorsal, lateral and sacroiliac ligaments. The muscle inserts into the femur and ribs. It enables the hip to extend and the limb to abduct.
The deep gluteus muscle originates from the superior ischiatic spine and ilium and is inserted into the femur. This muscle enables the limb to rotate inwards.
The coccygeus is at the top of the tail, followed by the inter transversal caude. The sacralcoccygeus ventralis and lateralis help to raise and move the tail.
Semitendinosus muslce begins at the transverse processes of the first and second coccygeal vertebrae, and the tuber ischium. The muscle is inserted into the cranial side of the tuber calicos and the tibia. It flexes the knee joint, causing inward rotation of the leg and helps to extend the tarsus and the hip.
The adductor originates from the pubis and the ischium, inserting into the caudal side and the medial epicondyle of the femur. This muscle adducts the limb, rotates the femur and flexes the hip.
Rectus abdominus muscle runs from the sternum to the pubis behind. It forms the floor of the abdomen and is situated between the tendons of the internal oblique and the transversalis muscles.
Obliquus abdominis externus originates from the ribs and the latissimus dorsi tendon. The muscle fibres run in an oblique line down and back towards the haunches, inserting into the linea alba in the centre, the pubis and the ilium. The muscle helps to flex and arch the horse’s back and supports expiration. The muscle also helps to compress the abdomen so that the horse can urinate, defacate and give birth, if a mare. The obliques abdomens internus is a similar shape to externus, including the tendon. The muscle fibres run from behind, down and back, crossing the fibres of the external oblique muscle. The muscle originates from the ilium and Poupart’s ligament, which forms the ring through which the scrotum fit and inserts into the symphysis pubis, the linea alba and the ribs. The action matches that of the external oblique muscle.
Rectus femoris muscle is part of the quadriceps femoris. It covers the front of the femur. It originates via two short tendons: one each from the upper and under surface of the ilium, in front of the hip joint. It inserts into the upper and anterior of the patella. The muscles helps to flex the femur and extend the tibia on the stifle. The vastus lateralis, intermedius and medialis are also part of the quads.
The brachialis muscle originates in the humerous and inserts in the proximal radius.
The external intercostal muscles are the small muscles in-between the ribs and are used in respiration.
Anatomy of the underside of the neck of the horse
The sternothyoideus and the sternohyoideus originate at the sternum cartilage, inserting into part of the larynx and the hyoid bone.
The longus capitis and longus colli originate from cervical vertebrae and the first 5-6 thoracic vertebrae.
The intertranswersarii are small muscles which are placed between the vertebrae transverse processes.
The scalenus originates on the cranial and lateral side of the first rib and is inserted into the transverse process of numbers 4-7 of the cervical vertebrae.
The subclavius is a cranial pectoral muscle, originating on the sternum and the cartilage of the first four ribs. It inserts on the scapula and helps to draw the front leg back or pull it in towards the body. It can also move the body forward over the weighted leg.
The caudal deep pectoral muscle, is the largest pectoral muscle. It originates on the sternum and is inserted in the humerous and a tendon.
The supraspinatus originates below the trapezius and ends at the scapula. It helps to maintain the shoulder in extension.
Equine Muscle Anatomy Part 1 explains the superficial muscles of a horse.