I. MUSCLES OF THE HEAD.

I. Muscles of the Eye.

1. M. rectus inferior (Fig. 48 ri′).

This muscle arises by a very thin, almost thread-like tendon from the parasphenoid, near the foramen opticum, and becomes rapidly broader; it lies beneath the tendon of the membrana nictitans and a part of the m. retractor bulbi: then runs forwards and outwards to be attached into the lower part of the circumference of the eyeball.

2. M. rectus externus (Fig. 48 re).

Arises by a thin, thread-like tendon from the parasphenoid near the foramen opticum, internal to and a little behind the m. rectus inferior: it then passes obliquely outwards over the m. retractor bulbi and the tendon of the membrana nictitans, to be inserted into the outer and hinder part of the circumference of the eyeball.

3. M. rectus internus (Fig. 48 ri).

A long, thin muscle, which arises by a thread-like tendon from the parasphenoid at the inner and hinder angle of the orbit, passes along its inner wall, and is inserted into the antero-internal part of the circumference of the eyeball.

4. M. rectus superior (Fig. 49 rs.).

Arises by a slender tendon from the fronto-parietal bone, widens rapidly, and runs forwards and outwards to be inserted into the upper part of the circumference of the bulbus.

5. M. obliquus inferior (Fig. 48 oi).

This muscle arises near the floor of the orbital cavity, at the inner and anterior angle, from the palatine bone by a thin and long tendon, runs under Harder’s gland outwards and backwards, and is inserted, almost at a right angle with the axis of the eye, into the anterior part of the circumference of the eyeball.

6. M. obliquus superior (Fig. 48 os).

The tendon of this small muscle arises from the inner end of the palatine bone, near the origin of the m. obliquus inferior, runs upwards, backwards, and outwards, and passes into a flat muscle, which rapidly becomes broader, and is inserted into the upper surface of the eyeball.

7. M. retractor bulbi (Figs. 48 r, 50 r, r′).

This muscle surrounds the optic nerve, and is situated within the cone formed by the four recti muscles; it consists of three portions, which take their broad and fleshy (the recti and obliqui arise by thin tendons) origins from the under surface of the parasphenoid. The first portion (Fig. 50 r) arises from the under surface of the parasphenoid, nearly as far as the middle line, is broad and fleshy, and covers, from below, the tendon of the rectus externus, then runs forwards and outwards. The second portion (r′), which lies over this and the tendon of the m. rectus externus, has also a broad and fleshy origin, but runs more directly forwards, slightly crossing the preceding. The third portion arises above the two others by a strong thin tendon from the parasphenoid: it runs almost in the direction of the m. rectus internus, but more directly forwards. The first two portions pass more to the upper surface of the eyeball, the third more to the under surface; consequently the whole muscle forms a sheath surrounding the eyeball. The fibres to the upper surface of the eyeball are inserted somewhat in front of the equator of the eyeball, those to the under surface somewhat behind it.

8. M. levator bulbi, Dugès (Figs. 48 l, 51 l, 52 l).

There is no bony floor to the orbital cavity, and the oral and orbital cavities are only separated by soft tissues; these are:—the mucous membrane of the mouth, a thin fascia, and above this the muscle under consideration. The fibres of this muscle run from before and from the inner side obliquely backwards and outwards, below all the remaining muscles of the eye, with the exception of the origin of the m. retractor bulbi, and the tendons of the m. obliquus inferior, and the m. pterygoideus; they arise on the inner side from the upper and outer border of the fronto-parietal bone (Fig. 51), also from the transverse portion of the parasphenoid and pterygoid bones, anteriorly from the sphenethmoid and palatine bones; they run first downwards, then, in the direction indicated, underneath the eye; and are inserted externally by means of tendinous fibres into the superior border of the upper jaw.

9. From the foregoing muscle certain fibres pass to the lower eyelid, which may be regarded as a depressor palpebrae inferioris (Fig. 52 d.p).

Manz‍42 has described a muscle as depressor palpebrae inferioris which arises in the neighbourhood of the external angle of the eye, somewhat behind and below the lower border of the orbit, thence passes obliquely upwards to be inserted into the lower eyelid. This description is right as regards the insertion, but needs correction as regards the origin. The muscle is attached to the orbital border in so far that the tendinous expansion, into which the levator bulbi passes at its external border, is attached into the upper jaw; this bundle splits off from the upper surface of the levator bulbi at its middle, and runs outwards on the under surface of the eyeball to the hinder part of the lower eyelid.

10. Tendon of the membrana nictitans‍43 (Fig. 53 n′). The pigmented free border of the nictitating membrane passes, at the inner and outer angles of the eye, into a tendon which passes for some distance through fibrous tissue, then descends to the under surface of the eyeball and joins that of the opposite side, so that the free border of the eyelid, together with this tendon, form a complete ring. The tendinous part of the ring, which is thin and thread-like, is found on the under surface of the eyeball, and lies under the m. retractor bulbi, and is bound to this muscle by connective tissue. Manz is right in saying that the two muscles described by Dugès as levatores palpebrae inferiores (orbito-palpebral anterieur and posterieur, n. 5 and 6) do not exist; as is also the suggestion that he has confounded the tendon of the nictitating membrane with these; this is clearly shown in the figure in Plate VI of Dugès. With regard to the actions of these eye-muscles, it is plain that the four straight and two oblique muscles rotate the eyeball about three axes, one the axis of the eye, one vertical, and the third passing from behind forwards and inwards. The m. retractor pulls the eyeball into the orbital cavity backwards and downwards, while the levator raises it.

There is less agreement about the movements of the eyelids, or rather of the lower eyelid, for the upper has no independent movements. Dugès (l. c.) says that the two muscles, which he considers to be levatores palpebrae inferiores, and which, according to him, are connected with the m. retractor bulbi, draw up the lid as it is being drawn backwards and downwards by the latter muscle. The depression of the lid on the relaxation of the retractor and projection of the eyeball is due simply to elasticity. Manz‍44, on the contrary, has shown that the sinking of the eyeball by the contraction of the retractor must necessarily cause a rising of the nictitating membrane, as its tendons are attached to that muscle and so must follow its movements.

The depression of the lower lid occurs simultaneously with the raising of the eyeball by means of the m. levator bulbi, through the contraction of the m. depressor palpebrae inferioris, which proceeds from that muscle; this is easily understood, as they are but parts of the same muscle.

II. Muscles of the Face.

11. M. intermaxillaris s. dilatator narium (Fig. 54 im).

This small muscle lies in the space between the vertical processes of the two premaxillary bones, and consists of obliquely-crossing fibres arising from the one bone and inserted into the opposite one. It brings together the processes of the intermaxillary bones, so that their upper extremities approach each other. At the same time, the cartilaginous nasal coverings or wings (an) diverge and the nares expand; hence we may regard this muscle as corresponding to a dilatator narium.

12. M. lateralis narium (Ecker), (Fig. 54 ln).

This small muscle occupies the space between the anterior portion of the maxillary bone and the ascending process of the premaxillary bone. It arises from the maxillary, and ascends obliquely forwards to be inserted into the outer border of the ascending process of the premaxillary bone. It is antagonistic to the foregoing.

A third muscle is described by Dugès as sus-maxillo-post-nasal, n. 3 (nasalis externus, Klein); according to him it passes from the proc. nasalis of the maxillary bone to the outer border of the nasal opening, which it widens. According to Klein (l. c., p. 9), this muscle in R. temporaria runs to the upper border of the maxillary bone beneath the lower eyelid, and is long and narrow. I have, however, never been able to find muscular fibre in this situation.

III. Muscles of the Lower Jaw.

13. M. depressor maxillae inferioris, Carus, Zenker (Figs. 55, 56, 63 d.m.).

This is a strong, triangular muscle, wide above, pointed below. It is placed between the head and the shoulder-blade. It arises in two portions; the greater, from the fascia dorsalis (fd), covers the scapula, and passes inwards and backwards over the muscles of the back; anteriorly it passes over the m. temporalis and is attached to the fronto-parietal and squamosal, being continued to the fascia of the upper eyelid. The second part is smaller, and arises by fleshy fibres from the postero-superior arm of the squamosal, and from the posterior and inferior border of the cartilaginous tympanic ring. The two parts unite and are inserted into the hinder angle of the lower jaw. Cuvier correctly considers that only the anterior portion corresponds to the digastricus, the posterior answering to the m. cervico-maxillaris of snakes. It draws the lower jaw down and opens the mouth.

14. M. temporalis (Figs. 49, 52, 56, 57 t).

This muscle, together with the M. pterygoideus, occupies the space between the prootic and the eyeball; it arises by the greater part of its fibres from the upper surface of the prootic; it passes, narrowing, beneath the anterior arm of the squamosal, then over the pterygoid bone, between this and the maxillary and quadrato-jugal, receiving on the way other fibres which arise from the anterior border of the inferior arm of the squamosal and from the anterior circumference of the tympanic ring. The collected fibres end in a broad, flat tendon, which is inserted into the inner surface of the so-called proc. coronoideus.

15. M. pterygoideus (Figs. 49 p, 52, 57 pt).

This is partly covered by the foregoing, and lies between it and the eyeball. It arises at the inner wall of the orbit from the fronto-parietal, and from that portion of the prootic which may be regarded as the ala magna, above the m. retractor bulbi. The muscle is broad, and compressed from before backwards: it runs downwards covered by the m. temporalis, and soon passes into a long, thin tendon (Fig. 57 pt), which is inserted, behind the m. temporalis and immediately in front of the joint, into the inner surface of the lower jaw.

16. M. masseter (Figs. 56, 57 m).

Arises as a somewhat broad muscle from the horizontal process of the quadrato-jugal and is inserted into the outer side of the lower jaw, opposite the insertion of the m. temporalis.

17. M. submaxillaris (Figs. 58, 59, 61 sm, sm′).

This muscle forms the floor of the mouth as does the mylo-hyoideus in man; it is, however, not inserted into the hyoid bone.

It arises—1. From the whole of the inner surface of the upper border of the lower jaw‍45, except near the angle (Fig. 58 sm);

2. By a small portion (Figs. 58, 59, 61 sm′) near the skull, from the cartilaginous anterior cornu of the hyoid-bone which projects from the cartilaginous part of the prootic bone. This portion descends along the cornu to unite with the other portion. The two together form a membranous layer, the fibres of which run transversely and meet in a band of connective tissue along the middle line, forming a kind of linea alba. The sheath runs forwards to the most projecting part of the lower jaw superficially to the m. submentalis. Slightly in front of its posterior border the muscle is attached in its whole breadth to the skin of the throat by a fine lamella of connective tissue, which is not always complete. The posterior border of the muscle is attached to a fascia, which passes between the deeper muscles and is attached to these. It is known that this muscle plays an important part in the movements of respiration and swallowing; but its precise action has not yet been worked out.

18. M. submentalis (Fig. 59 smt).

This small muscle lies in the anterior angle of the lower jaw between the dentary bones. It consists of transverse fibres which pass from one bone to the other. Below it is covered by the most anterior portion of the m. submaxillaris.

Its action is to approximate the sides of the lower jaw: according to Dugès (l. c., p. 123), it assists in closing the nostrils, by drawing together the sides of the lower jaw and thus raising the premaxillae.

IV. Muscles of the Hyoid Bone and the Tongue.

19. M. geniohyoideus (Figs. 59 gh, 60 gh).

Arises from the lower jaw in two portions, the one from near the middle line above the m. submentalis, the other more externally from the upper border of the lower jaw. The two parts unite to form a long, flat muscle, which lies on the ventral surface of the body of the hyoid bone, and divides posteriorly into two portions. One of these, the median (Figs. 59, 60 gh″), is inserted into the inner border of the posterior horn of the hyoid bone, and is here attached to a fascia which covers the m. hyoglossus from beneath. By the same fascia the muscles of opposite sides are connected in the space between the two posterior cornua. The lateral division (Figs. 59, 60 gh′) is attached to the posterior cartilaginous process of the hyoid bone.

20. M. sternohyoideus (Figs. 59, 60 sh).

This muscle forms the cervical part of the m. rectus abdominis and is, in fact, a direct continuation of it. It arises by an inner portion (Fig. 59 sh″) from the upper surface of the inner extremity of the coracoid and from the xiphisternum, while the outer portion (sh′) is an immediate continuation of the m. rectus abdominis, which passes into this muscle at about the fifth inscriptio tendinea. The muscle passes forwards on the upper surface of the coracoid and of the clavicle, under the pericardium towards the hyoid bone; here the muscle, hitherto horizontal, suddenly becomes (compare Fig. 60) vertical, and passes between the two insertions of the m. geniohyoideus, and is inserted for a considerable length into the lower surface of the hyoid bone and the anterior extremity of the posterior cornu.

21. M. omohyoideus (Figs. 59, 60 oh).

Arises from the anterior border of the bony scapula, and is inserted into the outer part of the ventral surface of the body of the hyoid: it is partially covered by the m. sternohyoideus.

22. M. petrohyoideus anterior (Ecker), (Fig. 61 ph1).

A thin, flat muscle, narrow above, but broadening rapidly below, which arising from the outer extremity of the prootic bone, partially surrounds the pharynx below, into which some of its fibres are inserted; it is attached to the ventral surface of the body of the hyoid bone, near the lateral notch. The chief action of this muscle appears to be that of a m. constrictor pharyngis.

23, 24, 25. Mm. petrohyoidei posteriores (Ecker), (Figs. 61 ph 2, 3, 4).

These are three long, thin muscles, which lie nearly parallel to one another, and run from the prootic bone to the posterior cornu of the hyoid bone. All three arise, covered by the m. sternocleidomastoideus, from the lateral extremity of the prootic, pass round the pharynx and are attached, the m. petrohyoideus posterior primus (Fig. 61 ph 2) to the anterior extremity, the secundus (Fig. 61 ph 3) to the middle; and the tertius (Fig. 61 ph 4) to the hinder extremity of the posterior horn of the hyoid bone.

26. M. hyoglossus (Figs. 59, 60, 61 hg).

Arises from the bony posterior extremity of the hinder process of the hyoid bone (hg′). The muscles of the two sides pass forwards and converge to the middle line, where they unite. The azygos muscle thus formed runs forwards on the ventral surface of the hyoid bone, between the mm. geniohyoidei, over the anterior border of the hyoid, and passes into the tongue, where it turns backwards and runs to the tip. (For action, see tongue.)

27. M. genioglossus.

Arises above the insertion of the m. geniohyoideus on either side near the middle line from the two anterior bones of the lower jaw. The two origins quickly unite to form a thick, very firm muscular belly, which in its course backwards rapidly thins, and is distributed by numerous bundles; decussating with the fibres of the m. hyoglossus at an acute angle, it runs on to the anterior extremity of the tongue.

II. MUSCLES OF THE TRUNK.

I. Muscles of the Abdomen.

In the formation of the abdominal wall the following muscles take part:—(1) m. rectus abdominis, (2) m. obliquus externus, (3) m. obliquus internus, (4) the portia abdominalis of the m. pectoralis. Above, the m. obliquus internus forms a kind of diaphragm, partially separating the abdominal from the thoracic cavity (p. 70).

28. M. rectus abdominis (Fig. 62 r).

These muscles, situated on either side of the middle line of the abdomen, are as a rule traversed by five notched inscriptiones tendineae. Each muscle arises, by a narrow, strong tendon, from the inferior border of the pubes, passes forwards, and quickly widening, divides at the second inscriptio tendinea (counting from behind) into two portions.

(a) The Outer portion (Fig. 62 r) runs into the portio abdominalis of the m. pectoralis, and forms the greater lateral division of that muscle.

(b) The Inner (r′) continues as the m. rectus abdominis and gradually narrows anteriorly. At the sternum some of the fibres (the median) are inserted into the dorsal surface of the cartilaginous plate of the xiphisternum, while the greater part of the muscle continues forwards over the coracoid to become the m. sternohyoideus at about the fifth inscriptio tendinea (p. 64).

29. M. obliquus externus (Fig. 63 oe, oe′).

The m. obliquus externus arises from‍—

(a) The aponeurosis covering the long muscles of the back, and by this from the spinous processes. The anterior edge of the m. obliquus externus covers the posterior edge of the m. latissimus dorsi, which arises from the inferior, ventral surface of the aponeurosis. The aponeurosis also divides laterally into two parts, one of which, the posterior, passes into the obliquus externus, while the anterior forms the tendon of origin of the depressor maxillae inf. (dm) (compare Fig. 66).

(b) A second, narrow portion, portio omo-abdominalis (Fig. 63 oe′) (m. xipho-adscapulaire, Dugès, n. 62), arises from the posterior border of the scapula by a thin tendon, and becomes broader as it runs downwards and backwards, to join the anterior border of the other and larger portion.

The whole muscle is attached by its most anterior fibres to the cartilage of the xiphisternum, the rest passing into an aponeurosis which, inseparably connected with the inscriptiones tendineae, traverses the lower surface of the m. rectus abdominalis to the linea alba.

30. M. obliquus internus (and transversus), (Figs. 64, 65 oi).

This muscle corresponds with the combined obliquus internus and transversus, and has therefore been described either as the one or the other, by various authors.

The fibres arise tendinously‍—

a. From the transverse processes of the vertebrae from the fourth backwards, and from the fascia covering the mm. intertransversarii.

b. From the iliac bone, by a strong tendon from its upper border, and by a few weaker fibres from its outer surface. The latter fibres are covered by the former, and these end posteriorly in a sharp concave border.