MOEPHOLOGY OF THE TRUNK-MUSCLES. 333 



for the most pait in a longitudinal direction, as from spine to spine, in spinalis 

 and interspinales ; from transverse process to transverse process in longissimus 

 dorsi and rntertransversales ; or from rib to rib in ilio-costalis. Tlie second set 

 consist of muscular fibres dii'ected more or less obliquely upwards and outwards 

 from spines to transverse processes, as in splenius capitis and colli, rectus posticus 

 major and obliquus inferior. The third set are also oblique, but they are directed 

 upwards and inwards from transverse processes to spine, as in complexus, semi- 

 sptnalis, multifidus, and obliquus superior ; and from transverse processes to 

 laminae, as in rotatores costarum. 



In the posterior part of the trunk in tailed animals this dorsal series of muscles 

 is continued backwards without inteiTuption as the superior caudal muscles, and 

 in man an occasional muscle is sometimes found developed as an extensor coc- 

 cygis ; but as a general rule owing to the very slight development of the caudal 

 vertebrce, and the large size of the pelvic girdle, the dorso-lateral muscles do 

 not in man extend beyond the upper part of the sacrum. 



Anteriorly this muscle is prolonged to the side of the head, where it forms 

 'the group of temporal, pterygoids, masseter, and probably the orbital muscles. 

 The connection between these muscles and the dorsal muscles is well seen in the 

 Tailed-Batrachians, where the fibres of the dorsal muscle are directly continuous 

 with those of the temporal. In man this continuity is interrupted by the ossifi- 

 cation of the temporal ridge and root of the zygoma. 



The Tcniro-latcral muscle, while equally simple in the lowest vertebrates with 

 the dorso-lateral, presents in the higher animals much greater complexity lioth of 

 form and attachments. As regards its general relation to the vertebral axis of 

 the body it may be divided into two portions, which are usually quite distinct 

 from each other. 



The first or deeper portion is that to which the name of hupnx'ml may most 

 properly be applied. It consists of fibres which lie for the most part immediately 

 imder the bodies of the vertebra and attached to them. They may also 

 spread round the internal surface of the walls of the visceral cavity of the 

 body, and may even, as in the diaphragm and levator ani, spread across that 

 cavity, forming one or more muscular partitions. Posteriorly this set of muscles 

 is prolonged on the under surface of the taU, as the most deeply situated layer, 

 corresponding to an occasional muscle in man, the cuiwator coccygis. In man, 

 also, two prolongations are sent outwards on the hind limb, one on its preaxial 

 border, the psoas, the other on its postaxial, the pyriformis. Passing forwards, we 

 find belonging to the same gi'oup, the subvertebral muscles, represented in man by 

 the subcostals, but which in birds, serpents, &c., are very largely developed as 

 the retrahentes costarum and the levatores costanmi intemi ; also the various 

 diaphragms, including the post-cardiac or midriff, and the pelvic or levator ani. 

 In this list, also, should be placed the triangularis stemi, or, as it is called by 

 some, the subcostalis anterior. 



In front of the thorax these hypaxial muscles are prolonged in two sub-divi- 

 sions, of which the upper forms the recti antici and longus colli, and the lower 

 includes the muscles which connect together the various parts of the hyo- 

 branchial arches and jaws, and invest the buccal and pharyngeal cavities ; the 

 chief of these being the hj^oid and styloid muscles, the digastric, or at least its 

 posterior belly, the lingnial muscles, the buccinator and palatal muscles, and the 

 constrictors of the pharj-nx. 



The second or more superficial portion of the ventro-lateral muscle may be dis- 

 tinguished as pnrax'inl (Mivart). It arises in connection with the transverse 

 processes of the vertebrre, from their lower surfaces and tips and from the lateral 

 septa and general fascial investment. It thus at its place of attachment to the 

 vertebral axis separates the epaxial from the hypaxial sets of muscles. It is 

 further distingiiished by the disposition which certain of its sclerotomes show to 

 ossification in the ribs and the limb-girdles. These paraxial muscular bundles 

 form the superficial layer of muscles on the ventral surface of the tails of fishes, 

 &c., and almost the entire thickness of the muscular layer which bounds the 

 visceral cavity of all vertebrates. Their connection with the alimentary canal is 

 limited to its extremities where they form the external sphincters. 



