MORPHOLOGY OF THE TRUNK-MUSCLES. 349 



found, is only a partial layer. In man these layers are represented, the first three by the 

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

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



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

 is prolonged into the thorax by the triangularis sterni. The sternal and costal portions of 

 the diaphragm and the subcostals, small in man, but very largely developed in birds, 

 serpents, &c., as the retrahentes costarum and levatores costarum interni, must also be 

 referred to this layer, as well as a part at least of the levator ani or pelvic diaphragm. 



The muscles of the deep layer, together with the hypaxial group, are sometimes regarded 

 as forming a distinct set corresponding to the visceral musculature of the head, which they 

 resemble in being innervated by fibres (the non-gangliated splanchnic efferent nerves of 

 Gaskell) of rather smaller calibre than those supplying the skeletal muscles generally 

 (somatic muscles of Gaskell) ; but evidence is wanting to show that they differ in origin and 

 development from the rest of the ventro-lateral muscle (except in the case of the diaphragm), 

 and that they agree in these respects with the visceral head-muscles. 



The internal oblique is directly continuous with the lower internal intercostals, and the 

 external intercostals and levatores costarum, as well as the serrati postici, are differentiated 

 portions of the same layer. In series with these are the scaleni and anterior intertransverse 

 muscles in the neck, and the quadratus lumborum and lateral intertransverse muscles in the 

 loins. Inferiorly the middle layer furnishes the coccygeus, originally the ischio-caudal or 

 abductor caudae muscle, and possibly also part of the levator ani 



The external oblique layer is prolonged upwards upon the side of the chest, and outwards 

 upon the upper limb as serratus magnus with levator anguli scapulae and rhomboidei, 

 pectorales, and latissimus dorsi, and between the limb and head as sterno-cleido-mastoid 

 and trapezius. The superficial perineal muscles also belong to this layer. They may be 

 derived from a sphincter cloacaa, which becomes divided into a posterior portion (sphincter 

 ani externus) and an anterior portion (bulbo-cavemosus) with the separation of the uro- 

 genital passage from the intestinal canal : the ischio-cavernosus and transversus perinei are 

 detached portions of the anterior division (Gegenbaur). 



The ventral or visceral musculature of the head exhibits a special segmentation in accord- 

 ance with that of the visceral portion of the head-skeleton, and the muscles are for the most 

 part arranged in groups corresponding to the cephalic visceral arches of the embryo, in which 

 they are developed. In connection with the mandibular arch there are two such groups of 

 muscles, all of which are supplied bj the fifth nerve. The one, placed more dorsally or 

 laterally, includes the muscles of mastication, temporal, masseter and pterygoids, with the 

 tensor palati lying on the anterior or preaxial side of the Eustachian tube (first visceral cleft) 

 and the tensor tympani. The other, occupying a ventral or mesial position, is differentiated 

 into mylo-hyoid and anterior belly of the digastric, which, acquiring an attachment to the 

 hyoid bone, cover over the prolongation of the longitudinal trunk-muscles in the floor of the 

 mouth. The muscles of the hyoid arch are the stylo-hyoid, posterior belly of the digastric, 

 and stapedius, all supplied by the facial nerve. The levator palati, arising on the hinder or 

 postaxial side of the Eustachian tube, and the palato-glossus correspond in position to the 

 inner side of this arch, but by innervation they are associated with the pharyngeal muscles. 

 In the third arch is developed the stylo-pharyngeus, supplied by the glosso-pharyngeal nerve ; 

 while the fourth does not appear to furnish any corresponding muscle. The constrictors of 

 the pharynx result from the differentiation of the dorsal muscular wall of the cephalic 

 portion of the alimentary tube, and the palato-pharyngeus has a similar origin, being 

 developed in the hinder part of the palatine process by which the primitive mouth and 

 pharynx are subdivided. 



The fourth or most superficial layer of the ventro-lateral muscle, corresponding to the 

 panniculus carnosus of animals, seems to be mainly developed from the cutaneous surface of 

 the external layer. In man this layer exists only upon the surface of the head and neck, 

 and very slightly over the shoulder. In the neck it forms the subcutaneous colli or platysma 

 myoides, and those slight continuations downwards which are occasionally found upon the 

 surface of the pectoral and deltoid muscles. On the head it furnishes the epicranial muscles with 

 the intervening aponeurosis, the auricular and the facial muscles. Although thus widely dis- 

 tributed over the head and neck, this layer is originally derived from the hyoid arch, in which 

 region it first makes its appearance in the embryo, and whence it extends downwards and 

 upwards in the course of development. The various muscles resulting from its segmentation 

 are therefore all innervated by branches of the facial trunk, which is the nerve of the hyoid 

 arch, and the ramifications of which have spread over areas belonging to other nerves pari 

 passu with the expansion of the muscular sheet. (Rabl, " Ueber das Gebiet des Nervus 

 facialis," Anatom. Anzeiger, 1887.) 



The evolution of the complex system of superficial head-muscles in man and apes from a 

 simpler arrangement, such as is met with in lemurs, has been fully studied by G. Ruge. In 

 the latter animals the cutaneous muscular layer is double ; the more superficial is the platysma- 



