Muscular System (Part 3) - Muscles of the Thoracic Limb

Summary

This presentation segment delves into the anatomy of the thoracic limb muscles, distinguishing between intrinsic and extrinsic muscles and their roles around the shoulder, elbow, carpus, and digits. It highlights key muscles like the trapezius, rhomboid, serratus ventralis, and latissimus dorsi, detailing their functions and interactions. The discussion also covers muscles affecting the elbow, such as the triceps brachii and biceps brachii, emphasizing their extensor and flexor actions. The narrative concludes with a reminder of the multi-joint influence of these muscles, encouraging viewers to continue to the next module on pelvic limb muscles.

Takeaways

• 💪 The muscles of the thoracic limb are divided into intrinsic and extrinsic muscles, with intrinsic muscles located entirely within the limb and extrinsic muscles connecting different regions of the body.
• 🦾 Muscles can be further grouped by the joints they act upon, including muscles of the shoulder girdle, shoulder joint, elbow, carpus, and digits.
• 🏋️‍♀️ The trapezius muscle, a flat triangular muscle, swings the scapula forward and backward and is divided into two parts: the cervical (larger) and thoracic (smaller) parts.
• 🔍 Rhomboideus is a deeper, heavier muscle located beneath the trapezius and has three parts based on their location: capitis, cervicis, and thoracis.
• 🏗️ Serratus ventralis, a large fan-shaped muscle, forms a sling supporting the trunk between the thoracic limb, crucial for weight-bearing.
• 🏃‍♂️ Brachiocephalicus is the principal extensor of the shoulder, also acting as a flexor of the neck when the limb is weight-bearing, subdivided into clydobrachialis and clydoscephalicus.
• 🚶‍♀️ Latissimus dorsi is a strong flexor of the shoulder and assists in pulling the limb caudally or advancing the trunk when the limb is fixed.
• 🤸‍♂️ Triceps brachii is the strongest extensor of the elbow with four heads in dogs, including a long, lateral, medial, and accessory head, all inserting on the olecranon of the ulna.
• 🦵 Extensors of the carpus are found on the craniolateral aspect of the limb, including muscles like extensor carpi radialis and common digital extensor.
• 🤲 Flexors of the carpus and digits are found on the caudomedial side of the limb, including flexor carpi radialis and two digital flexors (superficial and deep digital flexor).

Q & A

What are the two types of muscles that make up the muscles of the thoracic limb?

The muscles of the thoracic limb are composed of intrinsic and extrinsic muscles. Intrinsic muscles lie completely within one region of the body, having their origin and insertion in the bones of the thoracic limb. Extrinsic muscles run from one region of the body to another.

How are the muscles of the limb further grouped based on their function?

The muscles can be grouped based on the joint of the limb they act with, which includes the muscles of the shoulder girdle, shoulder joint, elbow, carpus, and the digits.

What is the function of the trapezius muscle and how is it divided?

The trapezius muscle helps swing the scapula forward and backward. It is divided into two parts: the cervical part (trapezius cervicalis), which is larger, and the thoracic part (trapezius thoracis), which is smaller.

What is the serratus ventralis muscle and its role?

The serratus ventralis is the largest and most important muscle attaching the thoracic limb to the trunk. It is a large fan-shaped muscle that forms a sling supporting the trunk between the thoracic limb.

What is the main action of the brachiocephalicus muscle?

The brachiocephalicus muscle is the principal extensor of the shoulder area and acts as a flexor of the neck. It raises and advances the shoulder.

Describe the latissimus dorsi muscle and its action.

The latissimus dorsi is a wide triangular muscle and is a strong flexor of the shoulder. It also pulls the thoracic limb caudally or advances the trunk if the limb is fixed.

What are the two parts of the deltoid muscle and their respective locations?

The deltoid muscle has two parts: the spinal part, which is thicker and fusiform in shape, and the acromial part, which is thinner and strap-like. They fuse before inserting and act together as a single unit.

Which muscle is considered the strongest extensor of the elbow and what are its heads?

The triceps brachii is the strongest extensor of the elbow. It has three heads: the long head, which originates from the caudal border of the scapula, the lateral head from the lateral humeral diaphysis, and the medial head from the medial humeral diaphysis.

What is the role of the tensor fasciae antebrachii muscle?

The tensor fasciae antebrachii muscle tenses the antebrachial fascia. It assists the triceps brachii in the extension of the elbow through its fascial connections.

How does the location of the pectoral muscles relate to their function?

The pectoral muscles are strong adapters of the forelimb. The deep pectoral muscle (ascending pectoral) advances the trunk when the limb is fixed on the ground, while the superficial pectoral muscle, which is subdivided into a descending part and a transverse part, also plays a role in forelimb adaptation.

What is the main action of the superficial digital flexor muscle?

The superficial digital flexor is the only muscle that flexes the distal interphalangeal joint. It also secondarily flexes the more proximal joints of the digit and the carpus, and supports the fat lock.

Outlines

00:00

💪 Thoracic Limb Muscles Overview

This section of the presentation focuses on the muscles of the thoracic limb, which are categorized into intrinsic and extrinsic muscles. Intrinsic muscles have their origin and insertion within the thoracic limb, while extrinsic muscles span across different body regions. Muscles are further grouped based on the joint they act upon, such as the shoulder girdle, shoulder joint, elbow, carpus, and digits. Each group is then subdivided into extensors, flexors, adductors, and abductors. The paragraph introduces the muscles acting on the shoulder girdle, including the trapezius muscle, which is divided into cervical and thoracic parts, and the rhomboidus muscle, which is composed of three parts based on their location. The serratus ventralis muscle, the largest and most important muscle attaching the thoracic limb to the trunk, is also discussed, along with the omotransversarius muscle.

05:01

🤚 Muscles Acting on the Shoulder Joint

The muscles acting on the shoulder joint are divided based on their primary action into extensors and flexors. The brachiocephalicus muscle, the principal extensor of the shoulder, is described, along with its subdivisions. The latissimus dorsi, a strong flexor of the shoulder, is mentioned, as well as the deltoid muscle, which has two parts that act together. The infraspinatus muscle is highlighted as a strong collateral ligament of the shoulder joint with multiple actions. The teres major and teres minor muscles are also flexors of the shoulder joint. The adductors of the shoulder, including the superficial and deep pectoral muscles, are discussed, along with the subscapularis and coracobrachialis muscles and their roles in shoulder stabilization and movement.

10:04

🤚💪 Muscles Acting on the Elbow and Carpus

This section discusses the muscles acting on the elbow and carpus. The triceps brachii, the strongest extensor of the elbow, is described, including its different heads and their origins. The anconus muscle, tensor fasciae antebrachii, and biceps brachii are also mentioned, with the latter being a primary flexor of the elbow. The paragraph continues to describe the muscles acting on the carpus, with the extensor carpi radialis being the largest and most prominent extensor. The common digital extensor, lateral digital extensor, and extensor carpi ulnaris are also discussed, along with the flexor carpi radialis and ulnaris. The digital flexors, including the superficial and deep digital flexors, are highlighted, with a focus on their role in flexing the distal interphalangeal joint and supporting the fat lock.

Keywords

💡Intrinsic Muscles

Intrinsic muscles are those that lie completely within one region of the body, having both their origin and insertion within that region. In the context of the video, intrinsic muscles are a subset of the muscles of the thoracic limb, which include muscles that are entirely contained within the limb itself, such as those that move the fingers or toes. They are crucial for the fine motor control and stability of the limb.

💡Extrinsic Muscles

Extrinsic muscles run from one region of the body to another, meaning they have their origin in one place and insert in another. In the video, these muscles are highlighted as playing a role in moving the limbs, such as the muscles that connect the thoracic limb to the torso. They are important for the overall movement and positioning of the limbs in relation to the rest of the body.

💡Thoracic Limb

The term 'thoracic limb' refers to the limbs of quadrupedal animals that are attached to the thorax, which includes the forelimbs of such animals. The video discusses the muscles associated with this limb, emphasizing the distinction between intrinsic and extrinsic muscles and their roles in movement and stability.

💡Trapezius Muscle

The trapezius muscle is a flat, triangular sheet of muscle that is divided into cervical and thoracic parts. It is described as helping to swing the scapula forward and backward. This muscle is a key component of the shoulder girdle muscles and plays a significant role in the movement of the shoulder blade.

💡Rhomboid Muscle

The rhomboid muscle is described as a heavier muscle that lies deep to the trapezius. It is composed of three parts: rhomboideus capitis, rhomboideus cervicis, and rhomboideus thoracis, each originating from different areas of the body. This muscle group is important for the stability and movement of the scapula.

💡Serratus Ventralis

The serratus ventralis is identified as the largest and most important muscle attaching the thoracic limb to the trunk. It is a large, fan-shaped muscle that forms a sling to support the trunk between the thoracic limb. This muscle is crucial for the connection and movement between the limb and the body's core.

💡Brachiocephalicus Muscle

The brachiocephalicus muscle is mentioned as a heavy muscle that extends from the head to the arm. It is the principal extensor of the shoulder area and also acts as a flexor of the neck. This muscle plays a dual role in both extending the shoulder and flexing the neck, highlighting its importance in the overall movement of the forelimb and neck.

💡Latissimus Dorsi

The latissimus dorsi is described as a wide, triangular muscle and is a strong flexor of the shoulder. It also pulls the thoracic limb caudally. Known as the broadest muscle of the back, it is significant for its role in shoulder movement and trunk advancement when the limb is fixed.

💡Deltoid Muscle

The deltoid muscle is mentioned as extending from the spine of the scapula to the deltoid tuberosity of the humerus. It has two parts that fuse and act together, playing a role in the abduction and stabilization of the shoulder joint. This muscle is essential for shoulder movement and arm elevation.

💡Triceps Brachii

The triceps brachii is identified as the strongest extensor of the elbow. It has three heads (long head, lateral head, and medial head) and an additional head in carnivores like dogs. This muscle is crucial for extending the elbow joint and is a primary muscle for supporting the limb in an extended position.

💡Biceps Brachii

The biceps brachii is described as assisting in holding the shoulder joint in position and may extend it to some extent, but its chief action is the flexion of the elbow. This muscle is a key flexor of the elbow joint and is an example of a muscle that can have different actions depending on the context of its use.

Highlights

The muscles of the thoracic limb are divided into intrinsic and extrinsic muscles.

Intrinsic muscles have their origin and insertion within the bones of the thoracic limb.

Extrinsic muscles run from one body region to another.

Muscles are further grouped based on the joint of the limb they act with.

The trapezius muscle is divided into cervical and thoracic parts.

The rhomboidus muscle is composed of three parts based on their area.

Serratus ventralis is the largest muscle attaching the thoracic limb to the trunk.

Oma transversarius assists in lateral flexion of the neck.

Brachiocephalicus is the principal extensor of the shoulder area.

Supraspinatus acts as a stabilizing ligament of the shoulder joint.

Latissimus dorsi is a strong flexor of the shoulder and pulls the limb caudally.

Deltoid muscle has two parts that act together as a single unit.

Infraspinatus acts as a collateral ligament of the shoulder joint.

Teres major is a strong flexor of the shoulder joint.

Adductors of the shoulder form the substance of the brisket and are divided into superficial and deep pectoral muscles.

Coracobrachialis is a small muscle that acts as an extensor of the shoulder joint.

Triceps brachii is the strongest extensor of the elbow.

Tensor fasciae antebrachii assists the triceps in the extension of the elbow.

Biceps brachii assists in holding the shoulder joint and flexes the elbow.

Flexors of the carpus include the flexor carpi radialis and flexor carpi ulnaris.

Digital flexors include the superficial and deep digital flexors.

Muscle actions are not exclusive to a particular joint; they can act on multiple joints.

Transcripts

welcome to part 3 of module 4.

this part of presentation will be

dealing with the muscles of the thoracic

limb

the muscles of the thoracic limb are

composed of intrinsic and extrinsic

muscles

as i recall an intrinsic muscle lies

completely within one region of the body

where they have their origin and

insertion in this case

the bones of the thoracic limb on the

other hand

extrinsic muscles run from one region

of the body to another moreover

the muscles can be grouped further based

on which joint of the limb they are

acting with

this include the muscles of the shoulder

girdle

muscles of the shoulder joint the elbow

muscles of the carpus and the muscles of

the digits

in addition the muscles within the

grouping are subgrouped further as to

whether they are

an extensor a flexor an adductor or

abductor let us begin with the muscles

acting on the shoulder girdle

the first muscle is a flat triangular

sheet of muscle as shown here

this is the trapezius muscle it is

divided into two parts

a cervical part or the trapezius

services

is larger compared to the thoracic part

or the trapezius thoracis

which is smaller this muscle helps swing

the scapula forward and backward

the next muscle is the rhomboidus if the

trapezius is removed or reflected

the rhomboidus muscle can be seen this

is a heavier muscle just dipped to the

trapezius

it is composed of three parts and were

named based on their area

at the neck region rhomboideus capitis

is a long

thin strap-like muscle that originates

on the occipital bone

rhomboideus services has more substance

and originates at the cervical rafi

rhomboideus thoracis is a short thick

expanse of muscle it originates on the

spinous process of the thoracic vertebra

four

to seven serratus ventralis is the

largest and the most

important muscle attaching the thoracic

limb to the trunk

it is a large fan-shaped muscle it forms

a sling that supports the trunk between

the thoracic limb

take note that in this image the

trapezius and the latissimus dorsi

muscles were reflected

to show the serratus ventralis muscle

oma transversarius is a separate muscle

of the shoulder region

it usually pulls the distal end of the

scapula forward

although with the limb in weight bearing

position it

instead assists a lateral flexion of the

neck

now let us discuss the muscles of the

shoulder joint

muscles under this section is divided

based on their main action

let us begin with the extensors first is

the brachiocephalicus muscle

from the name itself it extends from the

head or cephalic

to the arm or brachium this is a heavy

muscle

covering the cranial aspect of the point

of the shoulder

it raises and advances the shoulder it

is the principal extensor of the

shoulder

area and acts as a flexor of the neck

when the limb is weight bearing

this muscle is subdivided into

clydobrachialis

extending from the clavicular tendon to

the humerus

and clydoscephalicus extending from the

clavicular tendon to the head and neck

cephalicus may be further subdivided

into a mastoid part or the

clydomastoidus

attaching to the mastoid process of the

temporal bone

and a cervical part or the clydos

cervicalis

remember that the clavicular tendon

represents the vestigial clavicle

supraspinatus acts chiefly as a

stabilizing ligament of the shoulder

joint

this is a deep muscle attached to the

supraspinous fossa

for the flexors of the shoulder joint

let us begin with the latissimus dorsi

this is a wide triangular muscle and is

a strong flexor of the shoulder

also it pulls the thoracic limb caudally

or if the limb is fixed

it advances the trunk it is also known

as the broadest

muscle of the back the next flexor is

the deltoid use muscle

this muscle extends from the spine of

the scapula

to the deltoid tuberosity of the humerus

it has two parts which

fuse before they insert and act together

as a single unit

the scopular part is the thin and

shit-like

while the acromial part is a thicker and

is fusiform in shape

the infraspinatus muscle acts as a

strong collateral ligament of the

shoulder joint

and may abduct flex and outwardly rotate

the shoulder

this muscle is located filling the space

of the infraspinous fossa

the teres major as shown here is also a

strong flexor of the shoulder joint

the brachium is removed to show more of

the infraspinatus

and the teres major muscles also part of

the flexor of the shoulder

is the teres minor muscle this muscle

lies just distal to the infraspinatus

muscle

and has the same action with the

infraspinatus

now let us discuss the adductors of the

shoulder

muscles form the substance of the

brisket

commonly they are divided into the

superficial pectoral muscle

and the deep pectoral muscle the deep

pectoral muscle is also termed as the

ascending pectoral muscle because of the

ascending fashion of the muscle fibers

on the other hand the superficial

pectoral muscle

is further subdivided into a descending

part

and a transverse part the pectoral

muscles

are strong adapters of the forelimb and

the deep pectoral muscle also advances

the trunk when the limb is fixed on the

ground

to see other adapters of the shoulder

here is a medial view of the scapular

region

the subscapularis muscle stabilizes the

shoulder on the medial side

this muscle fills the space of the

subscapular fossa

the coracobrachialis on the other hand

is a small muscle

in which the location of the muscle

belly suggests

that it is a shoulder flexor but its

attachment makes it a muscle

of an extensor of that joint

now let us move to the muscles acting on

the elbow

since the elbow is a hinge joint the

muscles acting on it

are either flexors or extensors in

general

the extensors are located at the lateral

side while the flexors are seen at the

medial side of the limb

in quadrupeds the extensors are stronger

than the flexors because they support

the weight of the body

by maintaining the limb in extension

position

the first muscle is the triceps brachai

muscle

it is the strongest extensor of the

elbow

it fills the space between the scapula

and the humerus

from the name itself it has three heads

however in dogs

foreheads are present shown here is the

long head

which originates from the caudal border

of the scapula

the lateral head which originates from

the lateral

humeral diaphysis

to see the other heads of the triceps

brachii here is a dissected arm

showing the medial and lateral side for

orientation purposes

here is the long head and the lateral

head

which were previously discussed take

note that the lateral head is reflected

this time

the medial head originates from the

medial humeral diaphysis

the fourth and the additional head in

carnivores like dog is the accessory

head

it originates from the humerus between

the medial and the lateral heads

take note that all heads inserts on the

olecranon process

of the ulna

deep to the triceps brachii is a rather

small muscle called anconus

it covers the caudal aspect of the joint

capsule of the elbow

next is the tensor fasciae antebrachii

the muscle's name reflects its action on

the

antebrachial fascia or it tenses it

but through these fascial connections

the tensor fasciae antibrachi also

assist the triceps

in the extension of the elbow

as mentioned most of the flexors are

located at the medial side of the limb

one of the flexors of the elbow is the

biceps brachii

this muscle assists in holding the

shoulder joint in

a position and may extend it to some

extent

however the chief action is flexion of

the elbow

also classified as flexors are the

brachialis and the pronator teres muscle

the corpus and digits like the elbow act

essentially as a hinge joint the

extensors of the carpus

lie on the craniolateral aspect of the

limb while the flexors are found on the

codomedial side of the limb

here are the muscles of the antibrachium

in a lateral

cranial medial and cuddle view for

better visualization

take note that the muscle action is not

strictly on a certain joint

for example a muscle can be an extensor

of the corpus but

a flexor of the elbow what we will at in

this presentation

is their main action the first muscle is

the

extensor carpi radialis this is the

largest extensor of the carpus

this is also the most prominent muscle

on the front of the forearm and is the

most cranial muscle of the group

the common digital extensor is the

longest extensor muscle in the thoracic

limb

this muscle is an extensor of all joints

of the digit

including the fat lock it may also

assist in extending the carpus

and even in the flexing of the elbow

the lateral digital extensor originates

on the lateral collateral ligament

of the elbow and lateral radius and ulna

the insertion varies according to the

number of digits present

in the case of dog it has five

insertions

the extensor carpi ulnaris is formerly

known as the ulnaris lateralis

it is the most cuddle of the extensor

muscles

in most domestic animals this muscle

probably produce a weak flexion of the

carpus

although by origin and nerve supply it

belongs to the extensor group

it also produces some outward rotation

of the forearm

the extensor carpi oblicus or the

abductor police's longus or the abductor

digit 1

is a flat triangular extensor of the

carpus

lying deep to the digital extensor

muscles of the antebrachium

as you observed most of the extensors

are seen at the lateral side of the

antebrachium

for the flexors same views of the

antebrachial muscles are shown

let us first locate the flexor carpi

radialis

this is caudal to the radius which is

palpable directly beneath the skin

the flexor carpi ulnaris exerts

considerable leverage

as a flexor of the carpus by inserting

on the accessory carpal bone

it projects in a pulmonary direction

from the lateral aspect of the carpus

these muscles are of course primarily a

flexor of the carpus

but they may act slightly in the

extension of the elbow

there are two digital flexors here is

the superficial digital flexor

and a deeper dissection is needed to

expose the deep digital flexor as shown

here

it lies the closest to the metacarpal

bone

it is the only muscle that flexes the

distal interphalangeal joint

secondarily it also flexes the more

proximal joints of the digit

and the carpus it is also important in

supporting the fat lock

and that concludes our presentation on

part three of module four

just a point to remember the action of

each muscle is not exclusive to a

particular joint

it can be an extensor of a proximal

joint

but a flexor of a more distal joint

you may now proceed to part 4 of this

module which is focus on the muscles of

the pelvic limb