The muscular bundles diverge from these points, some running forwards, some backwards; the former are only partially covered by the portio omo-abdominalis of the m. obliquus externus (compare Figs. 62, 63), in front of the anterior edge of that muscle. The posterior border of the muscle is uncovered.

The insertions of the muscle are very various, and as a consequence very various functions are accomplished by it.

a. The most anterior part of the muscle (Fig. 65 oi‴), the fibres of which run forwards, is attached‍—

1. Partly to the xiphisternum and the coracoid.

2. A second portion, viz. that arising from the transverse processes of the fourth vertebra (Fig. 64 oi″), surrounds the pharynx like a diaphragm (Fig. 65 oe), and is attached to its side as far as the dorsal aspect (Fig. 65 oi′). This portion is lightly separated from the following.

3. A third portion, placed behind the preceding, runs from the pharynx over the pericardium (Fig. 64 pc), and is attached to this nearly as far as the middle line (Figs. 64 oi′ and 65 oi″), resting on the sternum, the m. rectus and m. sterno-hyoideus. The lines of insertion of the muscles of opposite sides form an angle, open in front. In the thin borders of this portion the muscular bundles lie almost entirely in simple layers, and they are therefore well adapted for microscopical purposes.

b. The middle and posterior portions of the muscle pass downwards and slightly backwards, towards the outer border of the m. rectus; there they pass into a tendon which, for the most part, runs on the upper surface of the rectus to the linea alba.

II. Muscles of the Back.

The muscles of the back are covered by a fascia, the fascia dorsalis (Fig. 66 fd), attached to the spinous processes of the vertebrae. Anteriorly, where it covers the m. temporalis, it is attached to the fronto-parietal and squamosal bones, posteriorly to the superior border of the iliac bones and the extremity of the urostyle. The posterior part, which covers the origins of the m. longissimus dorsi, of the m. coccygeo-iliacus, and of the coccygeo-lumbaris, is especially strong. From the anterior part several muscles arise laterally, viz. a portion of the m. obliquus abdominalis externus, of the m. latissimus dorsi, and of the m. depressor maxillae inferioris. In the uppermost layer, immediately covered by the fascia dorsalis, lie the m. cucullaris, the m. retrahens scapulae, the m. latissimus dorsi arising from the fascia itself, and the scapula with its muscles; then come the long muscles of the back; and in the third and deepest layer the short muscles of the back.

The individual muscles are as follows:‍—

A. Limb Muscles of the Back (for moving the shoulder-girdle and arm).

31. M. cucullaris (Fig. 66 c).

In order to see this muscle, it is necessary to separate the fascia dorsalis from the spines of the vertebrae and to turn it back with the m. depressor maxillae inf. arising from it. It is then seen to rise from the posterior surface of the exoccipital as far as the middle line; the muscles of the two sides run backwards, diverging each from the other, so as to form an angle, and are then attached on each side to the anterior superior angle of the suprascapula at its under surface and median border.

Cuvier considers this muscle as the analogue of the m. levator anguli scapulae. Dugès and Meckel consider it to be the anterior part of the m. cucullaris, the former thinking the posterior part of the muscle to be the m. rhomboideus. I cannot agree with these opinions, and I regard the muscle as simply representing the m. cucullaris of man (Ecker).

32. M. latissimus dorsi (Figs. 56, 66 ld).

This thin, triangular muscle arises from the ventral surface of the fascia dorsalis, and is posteriorly covered in part by the anterior border of the m. obliquus abdominis externus. The muscle becomes narrower towards the sides, and passes into a flat tendon, which blends with that of the m. infraspinatus, and is attached to the outer surface of the crista deltoidea of the humerus by a triangular expansion.

33. M. retrahens scapulae (Ecker), (Fig. 66 r).

This flat, oblong muscle arises from the transverse process of the fourth vertebra, and is connected with the third tendinous inscription (counting from before) of the m. longiss. dorsi. Posteriorly its origin forms a convex border, whence it runs forwards and to the side; it is inserted into the ventral surface of the suprascapula, near its inner border. This muscle lies near the m. serratus or transv. scap. tertius (Fig. 68 ts″), and has an analogous position. It would, therefore, possibly be more correct to regard it also as a m. serratus. It arises from parts which represent ribs, and is inserted into the scapula. It has, at any rate, no analogy to the m. cucullaris, with which Dugès connects it, nor with the m. rhomboideus, which Klein holds it to be, as it does not arise from spinous processes.

B. Long Muscles of the Back.

34. M. extensor dorsi communis (Fig. 67 lg.d.).

This muscular mass is covered by the fascia dorsalis, the m. cucullaris and retrahens scapula, and the suprascapula: it corresponds with (1) the m. ileo-costalis and longissimus dorsi, dorsi and cervicis (Henle); (2) the semispinalis and spinalis (Henle).

The following separate portions may be made out:‍—

35. (1) M. longiss. dorsi (Fig. 67 lg.d.).

This muscle arises from the most anterior part of the urostyle, runs forwards near the middle line, separated from the muscle of the opposite side by the spinous processes of the vertebrae. The inner portion passes over the posterior spinous processes without being connected with them, as far as the fifth, it is then inserted into the succeeding spines as far as the exoccipital bone. The lateral portion runs outwards and forwards to be attached to the transverse processes and oblique processes of all the vertebrae, from the sixth forwards, while accessory muscular slips arise from the transverse processes and from the anterior spinous processes which, merging in the general muscle-mass, are inserted with this into the occiput.

The muscle is subdivided by a series of wavy, tendinous partitions (Fig. 67), which arise from the transverse processes, and traverse the muscle in planes, the upper surfaces of which are directed upwards and forwards. This arrangement of the muscle indicates its relation with the lateral trunk muscles of fish and fish-like amphibians.

36. (2) M. coccygeo-sacralis (Fig. 67 c.l.).

This muscle is partially covered by the preceding, it arises from the lateral surfaces of the anterior half of the urostyle; from this origin the fibres pass forwards and outwards to be inserted in two portions, the anterior set into the arch and the posterior set into the transverse process of the last vertebra. The former may be regarded as m. inter­cru­ra­lis, the latter as m. inter­trans­ver­sar­ius.

37. (3) M. coccygeo-iliacus (Fig. 67 c.i.).

This muscle arises from nearly the whole length of the lateral surface of the urostyle, the anterior portion being concealed by the preceding muscle. The fibres pass outwards and forwards to be inserted into the anterior two-thirds of the inner surface of the ilium.

38. M. ilio-lumbaris (Klein), (Fig. 67 il.).

Evidently represents the quadratus lumborum of human anatomy; it takes its origin from the anterior extremity of the ilium, and runs forwards to be inserted into the transverse processes of the vertebrae from the seventh to the fourth.

C. Short Muscles of the Back.

39. M. intertransversarius capitis superior (Ecker), (Fig. 67 i.c.s.).

A small muscle arising from the prootic: it lies above the levator anguli scapulae, and is inserted into the transverse process of the second vertebra.

40. M. intertransversarius capitis inferior (Fig. 67 i.c.i.).

This muscle lies on the ventral surface of the preceding one, but is easily separable from it; it arises from the angle of the prootic and is inserted, together with the preceding muscle, into the transverse process of the third vertebra. The n. vagus passes outwards between these two muscles.

41. MM. intertransversarii dorsi (Fig. 67 i).

Seven small muscles, placed between adjacent transverse processes of the vertebrae; from the second to the last, in continuity with the muscles last mentioned.

42. MM. intercrurales (Ecker), (Fig. 67 i′).

Those muscular fibres found between the arches of adjacent vertebrae are so named. On account of the shortness of the spinous processes they can scarcely be named m. interspinales.

III. MUSCLES OF THE ANTERIOR EXTREMITY.

I. Muscles of the Shoulder-girdle.

A. Dorsal Muscles of the Shoulder-blade.

(I.) Muscles arising from the head or trunk, and inserted into the scapulae, which they act upon primarily, and upon the shoulder-girdles secondarily.

Some of these muscles, such as m. cucullaris and retrahens, are situated on the back, and have already been described with the muscles of that region. Others are attached partly to the under-surfaces of the scapulae and are only visible from below; they are best seen in such a preparation as is shown in Fig. 68, made by cutting through the sternum in the middle line, and reflecting each half outwards.

(a) Muscles which arise from the skull.

43. M. levator anguli scapulae (Fig. 68 la).

A fairly strong muscle, which has a broad origin from the prootic and exoccipital bones as far as the foramen magnum, a few fibres are also attached to the lateral portions of the parasphenoid. The fibres run backwards and outwards, first in a frontal plane and then in a sagittal plane, to be inserted into the under surface of the suprascapula near the posterior border, where it encloses the hinder border of the m. interscapularis.

Its action is to draw the shoulders forwards and inwards, or the head downwards.

44. M. sternocleidomastoideus (Fig. 68 sc).

A narrow muscle placed in front of and externally to the preceding muscle; its general direction resembles that of the m. levator ang. scap.

The fibres arise from the most external part of the prootic and from the hindermost part of the squamosal, under cover of the m. digastricus maxillae, and from the hinder portion of the cartilaginous tympanic ring. Covered by these muscles, it passes backwards and downwards to be inserted into the concavity of the anterior border of the scapula; externally to the origin of the deltoid.

The action of this muscle is to draw the shoulder forwards and towards the middle line, or to bend the head downwards.

45. M. protrahens scapulae (Fig. 68 ps).

This muscle lies to the outer side of the rectus capitis inferior: it arises from the prootic, and runs backwards and outwards upon the levator anguli scapulae, to be inserted into the scapula.

It pulls the scapula forwards.

(b) Muscles which arise from the vertebrae.

46. M. transverso-scapularis major (Ecker), (Fig. 68 ts).

This muscle arises by tendon from the cartilage of the transverse process of the fourth vertebra and by a slip from the cartilage of the transverse process of the third vertebra. Diminishing in size, the muscle passes outwards, forwards, and upwards to be inserted into the posterior border of the scapula, opposite the insertion of the m. sternocleidomastoideus. It draws the shoulder inwards, backwards, and downwards.

47. M. transverso-scapularis minor (Ecker), (Fig. 68 ts′).

This is much shorter than the muscle just described; it arises under cover of the m. levator scapulae, from the transverse process of the third vertebra in front of the transverso-scapularis major; and runs outwards and forwards to be inserted into the ventral surface of the cartilage of the scapula. The action of the muscle is to draw the shoulder inwards, backwards, and outwards. Dugès regards this muscle as a portion of the serratus anticus magnus, the remainder of which he considers is to be found in the portio scapularis of the m. obliquus externus (xipho-adscapulaire, Dugès, n. 62).

48. M. transverso-scapularis tertius s. serratus (Ecker), (Fig. 68 ts″).

This is larger than the muscle just described; arising by a broad and flat origin from the transverse processes of third and fourth vertebrae, it runs upwards and inwards to be inserted into the ventral surface of the cartilaginous scapula near its inner border. In position, course, and action the muscle most nearly represents the m. serratus anticus of human anatomy.

(II.) Muscles on the scapula, that is, arising from the scapula and inserted into the scapula or humerus.

(a) Muscles on the deeper surface.

49. M. interscapularis (Fig. 69 i.s.).

One end of this muscle is tendinous, and is attached to the outer extremity of the suprascapula; from this the muscle, becoming fleshy and narrower, runs towards the middle line to become attached to the ridge on the ventral surface of the scapula, from which the m. deltoideus and the m. subscapularis arise, and between which it passes. It approximates the scapula and suprascapula and lessens the angle between them.

50. M. subscapularis (Fig. 70 ss).

This muscle is situated on the upper or visceral surface of the scapula. It arises from a bony ridge found on the upper surface of the coracoid and of the proc. coracoideus of the scapula. The muscle runs outwards and is inserted, after widening, into the inner surface of the crista deltoidea of the humerus; it is antagonistic to the deltoid, and draws the raised arm backwards and towards the trunk.

(b) Muscles on the superficial surface.

51. M. infraspinatus (Fig. 57, between ld and dm, Fig. 71 i).

The m. infraspinatus represents the m. infraspinatus, teres major and minor of human anatomy. It is of triangular form, with the broad base directed inwards; it arises from the whole of the upper surface of the suprascapula except along the inner border which is formed of hyaline cartilage alone: from this origin the fibres converge outwards to a flat tendon which unites with that of the m. latissimus dorsi to be inserted into the crista deltoidea humeri, by means of a thin, triangular, tendinous expansion.

General arrangement of the muscles of the shoulder-blade.

The muscles attached to the scapula and suprascapula are:

1. Muscles of the back (m. cucullaris and m. retrahens scap.).

2. Muscles of the neck (m. omohyoideus).

3. Muscles of the abdomen (portio scapularis of the m. obliquus externus).

4. True shoulder muscles (m. levator anguli scapulae; m. sternocleidomastoideus; m. protrahens scapulae; mm. transverso-scapularis major, minor, and tertius s. serratus; m. interscapularis; m. subscapularis; and the long head of the m. triceps).

From above the following are visible: m. cucullaris, a portion of the m. subscapularis, and the transverso-scapularis maior.

From below (after cutting through and drawing aside the two halves of the sternum) may be seen: the omohyoideus, sternocleidomastoideus, levator anguli scapulae, protrahens scapulae, transverso-scapularis minor and tertius, interscapularis and subscapularis.

B. Ventral Muscles of the Shoulder (Pectoral Muscles)

These arise from the sternum or shoulder-girdle and are inserted either into the humerus or into the radio-ulnar.

52. M. pectoralis (Fig. 72).

This is placed on the ventral surface of the shoulder-girdle and consists of the following parts:

(α) Portio sternalis anterior (Ecker), (Figs. 72, 73 p′).

This is the anterior portion of the m. pectoralis, it arises from the sternum proper and the epicoracoids. Broad at the origin, it becomes narrower as it runs outwards into a tendon attached to the crista deltoidea humeri.

(β) Portio sternalis posterior (Ecker), (Figs. 72, 73 p″).

This muscle is placed immediately behind the portio sternalis anterior, it has a somewhat broad origin from the sternum and xiphisternum; the muscle runs outwards and slightly forwards to be inserted into the groove beside the crista deltoidea humeri. The tendon of the m. sterno-radialis passes between these two portions of the m. pectoralis.

(γ) Portio abdominalis (Figs. 72, 73 p‴).

This portion represents that part of m. pectoralis major of human anatomy which arises from the costal cartilages, and more particularly that part which is connected with the aponeurosis of the m. obliquus abdominis externus. The muscle arises (1) from the m. rectus abdominis (Fig. 72 r), that is, it forms a direct continuation of the outer portion of this muscle; (2) the inner, smaller part arises from the superficial surface of the aponeurosis of the m. obliquus abdominis. The muscle becomes narrower as it courses outwards and forwards, the outer fibres being longer and more oblique than the inner. The action of this muscle will necessarily vary according to the part or parts which are brought into action; speaking generally, the anterior extremities will be drawn downwards and towards each other.

53. M. coraco-humeralis (Dugès), (Figs. 72, 73 c.h.).

A long, narrow muscle, lying upon the posterior border of the coracoid and the lowest part of the m. subscapularis. By its position it corresponds most nearly with the m. pectoralis minor, although its insertion does not. It has a narrow origin from the coracoid near the sternum, whence it courses outwards, under cover of the port. sternalis anterior and posterior of the m. pectoralis, to be inserted into the middle of the humerus between the deltoid and internal head of the triceps. It draws the limb towards the trunk.

54. M. sternoradialis (Cuvier), (Figs. 72, 73 s.r.).

This muscle evidently represents the biceps of man, it lies in front of the portio sternalis anterior of the m. pectoralis, its hinder border being under cover of the latter muscle. It takes origin from the episternum, omosternum, and the epicoracoid; from this broad origin the fibres converge while coursing backwards and outwards to be attached to a strong tendon. This tendon plays in a groove along the crista deltoidea, and is held in position by tendinous bands arising from the insertion of the m. pectoralis; it then pierces the lower portion of the muscular belly of the deltoid, and is inserted into the anterior extremity of the radial side of the radio-ulnar. The muscle is a powerful flexor of the forearm.

55. M. deltoideus (Figs. 70, 72, 73 d).

It is placed in front of and external to the m. sternoradialis, and represents both the m. deltoideus and supraspinatus of man. The muscle has two points of origin:‍—

(a) Portio scapularis, the larger, external portion is attached to the outer end of the clavicle, and to the anterior process of the precoracoid, but it arises chiefly from the dorsal surface of the scapula (from the same ridge as the m. subscapularis), and lastly from the anterior border and ventral surface of the scapula. The fibres pass outwards over the shoulder-joint.

(b) Pars clavicularis (musc. cleido-humeralis, Klein) arises from the inner extremity of the clavicle and from the omosternum and joins the portio scapularis.

The muscle is inserted into the humerus, the under part of the portio scapularis being attached to the upper portion of the crista deltoidea; the outer fibres of this part, together with those of the pars clavicularis, are attached to the extremity of the crista deltoidea and to the inner surface of the humerus as far as the distal extremity; just before its insertion the muscle is pierced by the tendon of the m. sternoradialis. The deltoid draws the limb forwards.

II. Muscles of the Fore-limb.

A. Muscles of the Arm.

M. sternoradialis (biceps). See page 82.

56. M. triceps brachii (Fig. 73 t.r.).

This muscle lies on the upper or dorsal surface of the arm, and has relations similar to those of the corresponding muscle in man. The long head arises from the posterior border of the scapula at the upper border of the glenoid cavity, and is here attached to the capsule of the joint; the inner head arises from the upper and inner surfaces of the anterior half of the humerus as far as the extremity; the outer head arises from the outer surface of the humerus. Additional fibres, which may be regarded as a fourth origin (m. subanconaeus), arise from the upper surface of the hinder half of the humerus. The fibres from these several origins unite to form a strong muscle, which covers the upper, inner, and outer surfaces of the bone, and then passes into a tendon. This tendon is attached to the capsule of the elbow-joint and inserted into the proximal extremity of the radio-ulnar. A cartilage, representing the olecranon, is found in the capsule at the point of attachment of the tendon.

B. Muscles of the Forearm.

(I.) Muscles of the flexor surface.

The muscles of this surface are arranged in two groups, so as to form, at the elbow-joint, a triangular depression (plica cubiti), into which the tendon of the m. sternoradialis sinks.

A. Muscles of the Inner Group.

57. M. flexor carpi I s. radialis (Fig. 74 Fc).

This represents either the flexor carpi radialis longior or radialis brevior of man; it has a broad origin from the inner border of the humerus above the epicondylus medialis. It becomes narrower as it passes downwards to be inserted by a tendon into the os semilunare and naviculare. The muscle is much stronger in males than in females, and the crest from which it takes origin is correspondingly larger in the former sex (p. 42). Tendinous fibres connect it with the following muscles:‍—

58. M. flexor carpi II s. ulnaris (Fig. 74 Fc′).

This muscle represents the flexor carpi ulnaris or ulnaris anterior, and is close to the inner side of the foregoing. It arises from the epicondylus medialis, being connected by fibres with the flexor digitorum communis, and is inserted into the os naviculare.

59. M. flexor digitorum communis (Figs. 75, 76, 77 Fd).