THE TRUNK MUSCLES. 507 



3. Stylo-Glossus (Fig. 1339). 



The stylo-glossus is described with the tongue (page 1579). 



Variations. — The stylo-glossus is occasionally absent, and may in such cases be replaced 

 by a mylo-glossus, which arises from the inner surface of the angle of the mandible or from the 

 stylo-mandibular ligament and is inserted into the sides and under surface of the tongue. 

 This muscle is usually present in the form of some small bundles of fibres having the attach- 

 ments described. 



4. LiNGUALis (Fig. 1340). 



The HnguaUs is described with the tongue (page 1579). 



III. THE TRUNK MUSCLES. 



THE DORSAL MUSCLES. 



In employing the term dorsal to indicate a group of muscles it must be clearly 

 understood that the group does not include all the muscles which, in the adult con- 

 dition, are found upon the dorsal surface of the body. The term, so far as it has 

 a topographic significance, refers to a phylogenetic stage in which the muscles it is 

 intended to designate were the only dorsal muscles, and, as here employed, it indi- 

 cates only those muscles which are derived from the dorsal portions of the embryonic 

 myotomes and are supplied by the posterior divisions (dorsal rami) of the spinal 

 nerves. 



An examination of the muscles of the back readily shows that they consist of 

 two distinct sets. There is a superficial set, consisting of broad and flat muscles, 

 which are, with few exceptions, attached to the skeleton of the fore-limb, and a 

 deeper set, consisting of elongated and relatively thick muscles, whose attachments 

 are confined to portions of the axial skeleton. The muscles of the former set, which 

 may conveniently be designated the spino-hiimeral mtiscles, are all supplied by 

 branches from the ventral rami of the spinal nerves ; they have reached their 

 present position, in which they almost completely cover in the true dorsal muscles, 

 by a secondary migration from the more ventral portions of the trunk, and prop- 

 erly belong to the system of limb muscles, in connection with which they will be 

 described. . 



The true axial dorsal muscles are all included in the deeper set. Viewed from 

 the surface, they appear to form elongated columijs of muscle-tissue, extending con- 

 tinuously, more or less parallel with the spinal column, over considerable stretches 

 of the back ; but when the more superficial portions of the columns are removed, it 

 will be seen that the deeper portions are associated with the individual vertebrae, 

 their fibres possessing a more or less distinct segmental arrangement. The columns, 

 indeed, are to be regarded as formed by the fusion of a number of originally inde- 

 pendent muscle-segments, derived from the dorsal portions of a corresponding 

 number of myotomes, a mode of formation also indicated by the fact that the 

 columns are supplied by nerves from a greater or less number of successive spinal 

 nerves, from just as many, indeed, as there are myotomes entering into their 

 composition. 



Comparative anatomy demonstrates that the dorsal musculature may, further- 

 more, be regarded as consisting of two parallel portions or tracts, a median and a 

 lateral. The former portion, which includes the majority of the dorsal muscles, is 

 composed of those muscles which fundamentally arise from the transverse processes 

 of the vertebrae and are inserted into the spinous processes, and may therefore be 

 termed the transversa- spinal portion ; while the more lateral tract consists of mus- 

 cles which, taking their origin primarily from the transverse processes, are inserted 

 into the ribs or their homologues, and may accordingly be termed the transverso- 

 costal portion. A certain amount of overlapping of the median tract by the lateral 

 one occurs in man ; indeed, in the lumbar region the two tracts fuse to a certain 

 extent to form the sacro-spinalis ; but throughout the thoracic and cervical regions 

 they are fairly distinct. 



