MYOLOGY.
The branch of anatomy which treats of the muscular system is called
Myology.
Muscles are the active organs of motion, or locomotion, each being
separated from the other by a thin delicate membrane made up of
connective tissue, which forms a sheath for the muscle. A muscle is
divided into two parts, viz.: muscular and tendinous. The muscular
part is the larger of the two. It is sometimes called the belly of
the muscle or flesh. This part is known as muscular tissue, and has a
reddish, meaty color. At both ends of the muscle there is a tendinous
part, or the hard, white portion of the muscle which becomes attached
to the bone. All muscles are attached to two or more places of
different bones, and when contracted, the joints of the body are moved.
They are well supplied with nerves, which give strength and feeling,
and also well supplied with small blood vessels, from which the muscle
is fed. Muscles are found in separate groups, all of which have
different actions to perform. There are two kinds of muscles--voluntary
and involuntary. The voluntary muscles are under the control of the
will of the animal; example--the muscles of the head, neck, back, hip
and legs. The involuntary muscles are beyond control of the animal,
and will act even though the animal were asleep--such as those of the
heart, the large muscular curtain which separates the chest cavity from
the abdominal cavity, which is one of the great muscles of breathing;
also the muscles around the chest which assist in breathing.
[Illustration: PLATE II.
MUSCLES OF THE HORSE--SUPERFICIAL LAYER--AFTER MEGNIN.]
EXPLANATION OF PLATE II.
MUSCLES OF THE HORSE.
This illustration shows the superficial muscles of the body after the
skin and panniculus carnosis muscle has been carefully removed. This
muscle is spread over the greater part of the body, which is related
externally with the skin; internally with the superficial layer of
muscles. Its action corrugates the skin, and thus enables the animal
to expel or shake off insects and irritating bodies, its use being
thus protective to some extent; it also supports and binds down the
superficial muscles.
SUPERFICIAL LAYER.
The panniculus and tunica abdominalis are removed.
1. Abducens.
2. Retrahentes muscles.
2´. Attollens maximus.
2´´. Attollens anticus.
3. Temporalis.
4. Nasalis longus.
5. Orbicularis palpebrarum.
6. Levator labii superioris alæque nasi.
7. Dilatator naris lateralis.
8. Orbicularis oris.
9. Zygomaticus.
9´. Buccinator.
10. Depressor labii inferioris.
11. Masseter.
12. Levator humeri.
13. Trapezius cervicalis.
14. Trapezius dorsalis.
15. Latissimus dorsi.
18. Pectoralis parvus.
19. Pectoralis magnus.
20. Sterno-maxillaris.
23. Subscapulo-hyoideus.
24. Antea-spinatus.
25. Teres externus.
26. Postea-spinatus.
30. Caput medium of the triceps.
31. Caput magnum extensor brachii.
33. Extensor metacarpi magnus.
34. Humeralis obliquus.
35. Extensor pedis.
36. Flexor metacarpi externus and medius.
37. Flexor metacarpi internus.
46. Cervical
47. Dorsal serratus magnus.
48. Rhomboideus longus.
49. Superficialis costarum.
50. Splenius.
52. Intercostales.
54. Tensor fasciæ latæ.
55. Triceps abductor femoris.
56. Gluteus externus.
61. Biceps rotator tibialis.
62. Rectus femoris.
63. Vastus externus.
65. Gastrocnemius externus.
68. Flexor pedis perforans.
69. Peroneus.
70. Extensor pedis.
71. Flexor metatarsi.
72. Flexor pedis accessorius.
74. Obliquus abdominis externus.
VOLUNTARY MUSCLES.--The voluntary muscles are in groups. The first
muscle we will call the student’s attention to, after removing the
skin, is the panniculus carnosus (not shown in Plate I), which is a
thin muscle, and almost entirely covering the body, which is sometimes
accidentally removed by a careless person in skinning the animal. The
action of this muscle is to shake the skin when flies or other objects
bother the horse. This muscle is not shown in Plate I it having been
removed in order to show the more important ones.
HEAD MUSCLES.--At the head there is a group of muscles which assist in
chewing, or masticating, the food.
GULLET OR PHARYNX MUSCLES.--Around the throat is another set of
muscles, sometimes called the muscles of the gullet, or pharynx, which
assist in swallowing.
NECK MUSCLES.--The neck muscles are divided into two groups, one on
each side. The action of these is to raise and lower the head, also to
turn the neck and head from side to side.
MUSCLES OF THE BACK.--The muscles of the back are generally divided
into two groups, one above the spinal column and the other below. The
muscles above the spine assist the animal in running, jumping and
rearing. The muscles below the spine are sometimes called the psoae, or
lumbar, muscles, situated below the lumbar bones, or the bones of the
small of the back. The action of these muscles is to assist the animal
in getting up. These muscles are important, for when paralyzed the
horse cannot use his hindquarters. Below these muscles are the kidneys.
HIP OR GLUTEAL MUSCLES.--The muscles of the hip are very large, filling
in around the hip bones. The action of these is much the same as those
of the back, as they assist in jumping, running, rearing and in flexing
and extending the hind leg.
[Illustration: PLATE III.
MUSCLES OF THE HORSE--DEEP LAYER--AFTER MEGNIN.]
EXPLANATION OF PLATE III.
MUSCLES OF THE HORSE.
_Deep Layer._
1. Temporalis.
1. Stylo-maxillaris.
2. Rectus capitis anticus major.
3. Sterno-thyro-hyoideus.
4. Sterno-maxillaris.
5. The Trachea.
6. Scalenus.
7. Splenius.
8. Funicular part of ligamentum nuchæ.
9. Rhomboideus longus.
10. Cervical.
15. Costal serratus magnus.
11. Cartilage of prolongation.
12. Rhomboideus brevis.
13. Transversalis costarum.
14. Longissimus dorsi.
15. Serratus Magnus.
16. External intercostals.
17. Internal intercostals.
18. Rectus abdominis.
19 19. Pectoralis magnus.
20. Postea-spinatus minor.
21. Flexor brachii.
22. Humeralis obliquus.
22´. Caput parvum (of triceps extensor brachii).
23. Extensor suffraginis.
24. Extensor metacarpi magnus divided.
25. Extensor metacarpi obliquus.
25´. Its tendon.
26, 28. Flexor pedis perforans and perforatus.
29. Obliquus abdominis internus.
30. Gluteus maximus.
31. Erector coccygis.
32. Curvator coccygis.
33. Depressor coccygis.
34. Rectus femoris.
35. Vastus externus.
36. Part covered by triceps abductor.
37. Biceps rotator tibialis.
38. Gastrocnemius externus.
39. Plantaris.
40. Flexor pedis perforans.
41. Peroneus.
42. Flexor metatarsi.
43. Extensor pedis (cut across).
TAIL OR COCCYGEAL MUSCLES.--Here there are four that are important,
one situated on the upper side of the tail when straight out, the
action of which is to raise the tail; two, one on each side of the
tail, have the power of drawing the tail to either side; the fourth is
situated under the tail and is the smallest one of the four. Its action
is to draw the tail down.
ABDOMINAL MUSCLES.--The abdominal or belly muscles, are four large,
flat muscles on each side of the abdomen. The outer edge of these
muscles is attached to the outer ends of the false ribs, also to the
processes of the lumbar bones and the outer angles of the pelvic or
hip bones. They unite below to what is called the linea alba, a hard,
white fibrous cord. They pass back in the center of the belly and are
attached to the front of the pelvic bones, called the lower bones of
the pelvic cavity. About ten inches from where it is attached here,
passing forward, is a small slit or hole, which is called the navel,
or umbilical opening. Here the navel vessels pass in and out during
the foetus life, or before the colt is foaled. This is a point of
importance to note, for sometimes at the time of birth this opening
does not close and allows the bowels to come down and form what is
known as umbilical or navel rupture.
Before finishing the description of this group of muscles a very large,
important ligament should be noted, which is found spread all over the
abdomen of the horse. It is of a yellowish color and about one-eighth
of an inch thick, attached in front to the back of the breast bone and
to the pelvic bones behind. This is the first structure seen after
removing the skin from the abdomen. This ligament gives great support
to the organs contained in the abdominal cavity. The action, or uses,
of the abdominal muscles are to support the organs contained in the
abdominal cavity, to flex the back-bone and assist in passing of the
feces. In the mare these muscles assist in parturition, or foaling.
SHOULDER MUSCLES.--The shoulder muscles are very large and powerful.
There are only three of great importance. Two situated on the outside
of the scapula or shoulder blade are important, as they are muscles
affected in the disease called shoulder sweeny. The other important one
is that which passes down over the shoulder joint through the groove
or pulley-like surface on the humerus, or shoulder bone. This is a
long, powerful muscle, attached above to the lower end of the scapula,
or shoulder blade, passing down through the groove mentioned, and is
attached to the upper and front part of the radius or fore arm bone.
Its chief point of importance rests in its action in raising the front
leg, where it passes over the pulley-like surface mentioned, when it
becomes injured or diseased; it is the seat of shoulder joint lameness.
MUSCLES OF THE FRONT LEG.--Muscles of the front leg, from the shoulder
down, are divided into two separate kinds, the extensor and flexor
muscles. The extensor muscles are those which bring the leg forward.
These muscles above are attached to the bones around the elbow joint,
passing down in front of the arm bones. About three inches above the
knee they become changed into the tendinous part of the muscles, or
what is called the cords of the leg. Some of them are attached to the
bones about the knee joint, while others pass over the front of the
joint and are held down to their place by a band or ligament, forming
a loop, as it were, for the tendinous portion of the muscle to glide
into when the leg is in action. Each one of these loops through which
the muscles pass are supplied with a synovial membrane to secrete the
synovia, or oil, which lubricate it during action, the same as in the
joint. This is a point of importance, as sometimes, on account of
injury or strain of this part of the joint through which the muscles
play there may be found a small, puffy enlargement containing oil
secreted by the synovial membrane. This disease is called bursal
enlargement.
The flexor muscles are situated at the back part of the leg, attached
above to the back part of the elbow joint, passing downwards at the
back part of the leg. About two or three inches above the back part
of the knee joint they become tendinous, and from there down to the
back part of the coffin bone, where two of the principle muscles are
attached; these form what is known as the back tendons, or cords, of
the leg. Some of them become attached to the back part of the knee,
same as the muscles on the front part of the leg, while the other two
principal tendons pass through a loop formed by ligaments, the same as
those mentioned in the front part of the knee. In tracing these tendons
down from the knee to the fetlock, notice that they pass through
another larger loop or sheath formed at the back of the fetlock, where
some of the fibers are attached, while others continue down at the back
part of the pastern bones, and are attached to the os pedis or coffin
bone. These tendons are important, because when they are strained the
fact is spoken of as the strain of the back tendons. The action of
these muscles is to flex the leg, bend the knee, pastern joints and
fetlock.
MUSCLES OF THE HIND LEG.--These are also divided into two groups,
extensor and flexor. The extensor muscles are situated in front of
the hind leg. They are attached above, around the stifle joint, and
pass downward in front of the tibia, or thigh bone, one being attached
to the front part of the hock. The other passes through sheaths, or
loops, supplied by a synovial membrane, formed by ligaments, to hold
the muscles firm in front while the leg is in action. In tracing them
down, in front of the shin bone to the fetlock, note that they pass
through loops, or sheaths, and continue down in front of the pastern
bones to where they are attached. The action of these is to bring the
leg forward.
The flexor muscles of the hind legs are attached, above, around the
back part of the stifle joint. In tracing them down it will be found
that they become tendonous. Two of the principal ones pass down to
that part of the hock joint, which forms the point known as the cap.
These form what is called the gambe of the leg, and are partly attached
at the point of the hock, and other part passing down to the fetlock
joint through a loop, or sheath, along the back part of the pastern
bones, and are attached to the os pedis or coffin bone of the foot.
This muscle, from the hock down, forms one of the back tendons of the
hind leg. Another important muscle is found passing down underneath
those already mentioned, through a loop, or sheath, at the back part of
the hock, where it is supplied with a synovia sac. This is a point of
importance, because when it becomes sprained it is the seat of what is
called thoroughpin. It then passes down the back part of the shin bone
beneath the other tendons already mentioned, through the loop at the
fetlock to the back part of the os pedis or coffin bone, where it is
attached. The action of these muscles are to flex or bend the fetlock
and raise the hock joint in traveling.
INVOLUNTARY MUSCLES.--Involuntary muscles, or muscles not under the
control of the will. The first to notice are those of breathing or
respiration. They form a group situated about the chest in such a way
as to be the means of increasing or decreasing the size of the chest
cavity. When these muscles expand the chest cavity is enlarged, causing
the air to rush into the lungs, known an inspiration. On the other
hand, when these muscles contract the air is expelled from the lungs,
known as expiration.
The diaphragm is a muscular curtain which separates the chest from the
abdominal cavity. It also assists greatly in drawing the air in, when
it contracts. This muscle also assists in passing faeces, and in the
mare foaling. It separates the heart and lungs from the bowels, liver
and stomach. This muscle should be carefully examined by the students.
It can be seen by opening any dead animal.
There is one muscle which is both voluntary and involuntary. It is
situated in the penis, surrounding the urethra, or the tube, which
carries the urine from the bladder to the penis in the male animal.
Its action is voluntary while the animal is passing urine. It is
involuntary during sexual intercourse, forcing the semen down through
the penis.
[Illustration: PLATE IV.
DIGESTIVE AND MALE URINARY APPARATUS OF A HORSE--AFTER MEGNIN.]
EXPLANATION OF PLATE IV
DIGESTIVE APPARATUS OF THE HORSE
1. Mouth.
2. Pharynx.
3. Œsophagus.
4. Diaphragm.
5. Spleen.
6. Stomach (left sac).
7. Duodenum.
8. Liver (upper extremity).
9. Great colon.
10. Cæcum.
11. Small intestine.
12. Floating colon.
13. Rectum.
14. Anus.
15. Left kidney and ureter.
16. Bladder.
17. Urethra.
_a._ Hard palate.
_b._ Tongue.
_c._ Soft palate.
_d._ Trachea.
_e._ Pulmonary artery (divided).
_f._ Heart.
_g._ Posterior aorta.