348 MORPHOLOGY OF THE TRUNK-MUSCLES. 



rectus by the separation of the fibres attached to the atlas and axis respectively, and in the 

 obliquus by a connection being formed with the transverse process of the atlas, which 

 acquires a much greater prominence in mammals, and thus carries outwards the differentiated 

 obliquus superior and inferior, leaving the interval known as the suboccipital triangle. 

 (Chappuis, " Die morphologische Stellung der kleinen hinteren Kopfmuskeln," Zeitschr. f . 

 Anat. u. Entwicklgsgesch., 1876.) 



In the postaxial part of the trunk in tailed animals this dorsal series of muscles is con- 

 tinued backwards without interruption as the superior caudal muscles, and in man a muscular 

 slip is occasionally found developed as a sacro-coccygeus posticus ; but in general, owing to 

 the slight development of the caudal vertebrae, and the large size of the pelvic girdle, the 

 dorso-lateral muscles in man do not extend beyond the upper part of the sacrum. 



In the head the dorsal musculature is in great measure suppressed, in accordance with the 

 absence of mobility in the cranium, and the protovertebrae which have been observed in this 

 region in the early embryo for the most part disappear without forming muscles ; but in the 

 first three segments the protovertebras undergo development and give rise to the orbital 

 muscles. From the first muscle-segment are formed the muscles supplied by the third nerve, 

 viz., the superior, internal and inferior recti, the inferior oblique, and the levator palpebrae 

 superioris, which is a derivative of the superior rectus. The second segment furnishes the 

 superior oblique supplied by the fourth nerve, and the third segment the external rectus 

 supplied by the sixth nerve. 1 



The ventro-lateral muscle, while equally simple in the lowest vertebrates with the dorso- 

 lateral, shows in the higher animals a much greater degree of complexity, both of form and 

 attachments. It springs from the ventral surfaces and tips of the transverse processes of 

 the vertebrae, as well as from the lateral septa and general fascial investment ; and by means 

 of its connection with the last-mentioned structure the superficial portion of the muscle is 

 prolonged dorsally, and gains an attachment to the spinous processes, so that it covers the 

 dorso-lateral muscle almost completely. Ventrally, it extends round the visceral cavity to 

 the median line, where it is separated from the corresponding mass of the opposite side by 

 the haemal septum, and it thus forms the whole thickness of the muscular portion of the 

 body -wall. It comes into connection with the orifices only of the alimentary canal, where 

 portions of the right and left muscles become united and form the external sphincters. 



According to the direction of its fibres the trunk portion of this muscular mass may be 

 divided into two groups of muscles, a mesial with longitudinally directed fibres and a lateral 

 with obliquely or transversely directed fibres. 



The mesial group is represented in man by the rectus abdominis, the sterno-hyoid, sterno- 

 thyro-hyoid, and omo-hyoid (the posterior belly of which has, however, acquired an oblique 

 direction by extending its origin to the scapula), the genio-hyoid and genio-glossus. The 

 other tongue-muscles supplied by the hypoglossal nerve, although differing widely in direc- 

 tion, are probably derived from the same source. In fishes the oblique fibres are almost 

 entirely wanting, 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 present as a more or less developed thoracic prolongation of the rectus. In some 

 animals, as Lepidosiren, the oblique fibres of the abdomen are directly continuous with the 

 longitudinal, but in man greater differentiation exists, for the lateral muscles are merely 

 prolonged forwards as strong aponeuroses which form a fibrous sheath for the rectus on each 

 side of the middle line. In man these longitudinal fibres have little or no connection with 

 the muscles of the limbs, but in urodeles 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 vestigial muscle in man, but 

 which in marsupials and monotremes is extremely large ; and as rare varieties between the 

 lateral oblique muscles forming a lateral rectus, which consists of a few fibres running 

 between the lower ribs and the ilium, or over the thorax as a supracostal muscle. Posteriorly 

 again the longitudinal direction is maintained by certain fibres of the quadratus lumborum. 

 The longitudinal muscles probably lie in the same plane with the middle stratum of the 

 lateral muscles. 



The lateral group of trunk-muscles, distinguished by the oblique or transverse direction 

 of their fibres, is divisible usually into three or it may be into four layers. In the lowest 

 vertebrates this stratification does not occur, but in the higher animals it is coincident with 

 the differentiation of separate muscles. Of these layers three are very constant in their 

 relations and extent, but the fourth, which is the most superficial, though very constantly 



1 In the lower vertebrates some of the hinder protovertebrae of the head give rise to muscles con- 

 necting the shoulder-girdle with the cranium (see Vol. I, Embryology, p. 161), but this appears not to 

 be the case in mammals, where the somites that have been recognised in the occipital region are said to 

 disappear in the course of subsequent development. 



