334 MORniOLOGY OF THE TRUNK-MUSCLES. 



According to the direction of its fibres tlie trunk portion of tliis muscular mass 

 may be divided into two groups of muscles, a mitral with longitudinally directed 

 fibres and a Intcral witli more or less obliquely directed fibres. 



The Ti-nfml group is represented in man by the rectus abdominis, rectus 

 thoracis (an occasional muscle), and the sterno-mastoid, and repeats more closely 

 tlie ^;impl8 segmented condition of the dorsal muscle than is the case with th.e 

 lateral filires. In fishes the oblique fibres are almost entirely wantmg. and in 

 the lower vertebrates generally they are less developed than the longitudinal. 

 On the other hand in the higher forms, as in man. the oblique fibres are the 

 more important, the longitudinal fibres being in certain places (thorax) 

 absent altogether, or only occasionally present as rectus thoracis. Again, the 

 connection of the rectus thoracis with the sterno-mastoid is but rarely seen in 

 man. In some animals, as Lepidosiren. the ol)lique fil>res are directly continuous 

 with the longitudinal, but in man greater differentiation exists, for the lateral 

 muscles are merely prolonged forwards as strong aponeuroses which fomi a 

 filjrous sheath for the rectus on each side of the middle line. In man these 

 tongitudinal fibres have little or no connection with the muscles of the limbs, but 

 in IJrodelans they are continued outwards upon the ventral aspect of each limb 

 as part of the pectoralis major and gracilis. 



Longitudinal fibres are also found in the pyramidalis. a small rudimentary 

 muscle in man, but which in marsupials and monotremes is extremely large : 

 they are also fovind, but rarely, in man between tlie lateral oblique muscles 

 forming a laimil rcctiix, which consists of a few filsres running between the lower 

 ril)s and the ilium. Posteriorly ngain the longitudinal direction is maintained 

 by certain fibres of the quafkatus lumborum. 



The hitcrid group of trunk muscles, distinguished by the oblique direction of 

 their fibres, is divisible usually into three or it maj- be into four- layers. In the 

 lowest vertebrates this stratification does not occur, but in the higher animals it 

 is coincident with the dift'erentiation of separate muscles. Of these layers three 

 nre very constant in their relations and extent, but the fourth, which is the 

 most superficial, though very constantly found, is on the whole only a partial 

 layer. In man these layers are repi'esented, the first three by the abdominal 

 muscles, the external and internal oblique and transversalis respectively, and the 

 fourth by the platysma myoides. the facial, auricular and epicranial muscles. 



The transverse or deepest of these layers is represented by the transversalis 

 muscle, and according to Humphry and others by the triangularis stemi and the 

 subcostals. which, however, have already been described as belonging more pro- 

 perly to the hj7)axial set of muscles. 



The internal oblique is in series with the internal intercostals. levatorcs costa- 

 rum. anterior cervical and lateral lumbar intertransversales, and scaleni muscles. 

 It is also in most direct connection with tlie quadratus lumborum. Posteriorly 

 this layer furnishes the ischio-caudal (occasionally found in man), the erector 

 penis, compressor urethras, and transversus perinrei. Laterally it gives the costo- 

 scapular muscles, serratus magnus and levator scapulaj. and the costo-coracoid, 

 or subclavius. to the shoulder girdle. 



The external oblique laj-er is prolonged upwards iipon the side of the chest, and 

 outwards upon the fore limb as pectorales major and minor, latissimus dorsi, and 

 between the limb and the head, as cleido -mastoid and trapezius. 



The fourth layer, corresponding to the panniculus camosus of animals, seems 

 to be mainly developed from the cutaneous surface of the last or external oblique 

 layer ; it is also developed in close connection with the skin and fascial invest- 

 ment. In man this layer extends only upon the surface of the head and neck, 

 and very slightly over the shoulder. It forms the subcutaneus colli or platysma 

 niyoides. and those slight continuations downwards which are found upon the 

 surface of the pectoral and deltoid muscles. On the surface of the head this 

 lonns the epicranial inuscles. with the intervening aponeurosis, the auricular 

 and the facial muscles, except the orbicularis palpeVn-arum. All these muscles 

 lire attached to bone usually bj- one end only, the other being attached to the 

 i-kni or to the cartilage of som.e moveable structiire. but in some cases tliey may 

 reach to the deeper stmctures by both end.'*. Portions of this layer in animals 



