BACK. 



371 



rhomboid, and is so united with the serratus 

 magnus that Dumeril considers it a portion of 

 this muscle. The dissection of it in some 

 quadrupeds favours this opinion, but in man 

 it appears rattier in connexion with the rhom- 

 boid. 



This muscle is covered by the sterno-mastoid 

 at its upper part, then by the integuments, and 

 afterwards by the trapezius. It rests on the 

 splenius colli, cervicalis descendens, trans ver- 

 salis colli, serratus posticus superior, and lesser 

 rhomboid. 



This muscle pulls the superior angle of the 

 scapula upwards and forwards, and by rotating 

 that bone on the thorax becomes a depressor 

 of the shoulder-joint. The rhomboids act with 

 it in depressing the joint; but the inferior 

 portion of the serratus magnus is its direct 

 antagonist. When the trapezius acts with this 

 muscle, the scapula is drawn directly upwards. 

 If the scapula be fixed, this muscle will incline 

 the neck to its own side. 



This muscle undergoes many modifications 

 in the different families of the mammalia. In 

 sirni&e it is inserted into the spine of the sca- 

 pula, not into its angle. In carnivora and 

 rodentia it separates the two portions of the 

 trapezius, and is inserted near the acromial 

 end of the spine of the scapula. In the cat it 

 arises from the basilar process of the os occi- 

 pitis and from only one of the cervical vertebrae, 

 the atlas. In the horse it does not exist at all. 

 In the dolphin it forms a thin tendon which 

 spreads over the scapula. As to birds and rep- 

 tiles, it is replaced in them by other muscles. 



Third layer. Two very thin muscles, the 

 serratus posticus superior and serratus posticus 

 inferior, constitute the layer. 



The serratus posticus superior is quadrilateral . 

 It arises by a thin tendon from the lowest part 

 of the ligamentum nuebae, from the last cervi- 

 cal and the first two or three dorsal spines. 

 The fleshy fibres which succeed form a thin 

 plane, pass downwards and outwards, and are 

 inserted by four digitations into the superior 

 border and external surface of the second, third, 

 fourth, and fifth ribs, a little external to their 

 angles. 



This muscle is covered by the rhomboid, 

 the trapezius, and, when the shoulder is drawn 

 back, by the serratus magnus. Its origin is 

 united to the two former. It covers the splenii, 

 the longissimus dorsi, transversalis colli, sacro- 

 lumbalis and cervicalis descendens; while on 

 these it is tendinous; then it becomes fleshy 

 and covers the ribs and intercostal muscles. 

 Sometimes it has only three points of insertion. 

 Occasionally we find a bundle of fibres passing 

 from the upper part of this muscle along the 

 levator anguli scapulae to be inserted into the 

 transverse process of the atlas. 



This muscle elevates the ribs and expands 

 the thorax as in inspiration. It binds down 

 the muscles on which it lies, enabling them to 

 act with more effect. 



The serratus posticus inferior is very like the 

 last muscle, but a little broader and thinner. 

 It arises from the last two dorsal and first three 

 lumbar spines by a thin tendinous expansion, 



which is intimately connected with the tendon 

 of the latissimus dorsi, and often destroyed in 

 removing the latter. The fleshy fibres which 

 succeed pass upwards and outwards to be 

 inserted by digitations into the four lowest 

 false ribs. The uppermost digitation is the 

 largest, and is attached to the rib near its angle ; 

 the others become smaller as we descend, and 

 their insertions are more remote from the angles. 

 The lowest is connected with the cartilage of 

 the last rib. This muscle covers the longissimus 

 dorsi and sacro-lumbalis, the ribs and inter- 

 costals. It also covers the posterior tendon of 

 the transversalis abdominis, to which it is in- 

 separably united. 



This muscle draws down the ribs as in ex- 

 piration, and binds down the deep lumbar 

 muscles. 



A thin semitransparent fibrous layer, called 

 the vertebral aponeurosis, covers the spinal 

 muscles in the interval between the two ser- 

 rati. It is continuous with their adjacent edges, 

 and assists them in binding down the long 

 muscles of this region. The fibres of which 

 it is composed pass for the most part trans- 

 versely, from the spinous processes to the an- 

 gles of the ribs. 



These muscles are generally present in the 

 inferior animals, when ribs exist, and have no 

 peculiarity worthy of being noticed here. 



The splenii form the fourth layer. They 

 appear as one muscle, extending from the lower 

 cervical and upper dorsal spines obliquely 

 upwards, outwards, and forwards, to the head 

 and to the transverse processes of the superior 

 cervical vertebrae. Covered below by the 

 rhomboid and serratus posticus superior, higher 

 up by the trapezius and levator anguli scapulas, 

 and higher still by the sterno-mastoid, it is 

 only about the middle of their course that they 

 become distinct from each other, for they arise 

 as one. 



The splenius colli (or splenius cervicis) is 

 the inferior portion, not so thick or broad as 

 the superior, but of greater length. It arises 

 from the spines of the third, fourth, fifth, and 

 sixth dorsal vertebrae, and from the interspinal 

 ligaments, by tendinous fibres which are long, 

 and form an acute angle below. The flat band 

 of fleshy substance which proceeds from this 

 tendon passes upwards, outwards, and for- 

 wards, then divides into two or three fasciculi, 

 which are inserted tendinous into the trans- 

 verse processes of the two or three superior 

 cervical vertebrae, blended with the attach- 

 ments of the levator anguli scapulae and the 

 transversalis colli. 



The splenius capitis, the superior portion, 

 arises from the spines of the two superior 

 dorsal vertebrae and of the seventh cervical, 

 and from the ligamentum nuchae as high as the 

 fourth cervical. At the origin it is tendinous ; 

 it soon becomes fleshy, passes upwards, out- 

 wards, and forwards, to be inserted into the 

 back part of the mastoid process of the tem- 

 poral bone, and into the external part of the 

 depression on the occipital, between the supe- 

 rior and inferior transverse ridges. 



These two portions ought not to be con- 



2 B 2 



