COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 149 



scapula to the sides of the thorax and neck. Origin, by slips from the first nine 

 or ten ribs and the anterior part (called by R and J levator scapulae) from the 

 transverse processes of the last five cervical vertebrae; insertion, scapula near 

 the vertebral border; action, draws the scapula craniad, ventrad, and against the 

 thoracic wall. 



m) Serratus dor sails (posterior). The anterior part of this muscle arises by 

 a number of fleshy slips from the ribs near their angles. The short slips soon 

 pass into a thin aponeurosis which overlies the epaxial muscles of the thorax, and 

 which is fastened to the median dorsal line. The posterior part of this muscle 

 consists of a few slips lying under the latissimus dorsi and appearing like a forward 

 continuation of the internal oblique. These slips originate on the last ribs and 

 are inserted by means of an aponeurosis onto the median dorsal line. Action, 

 draw the ribs forward. 



n) Scalenes. Raise up the pectoralis muscles from the chest wall by passing 

 the fingers under them. If necessary their posterior parts may be cut into. 

 Several long muscles will be seen in the chest wall ventral to the origin of the 

 serratus ventralis and in front of the anterior boundary of the external oblique. 

 These muscles are scalenes; they originate on the ribs and pass forward in a 

 nearly straight course to be inserted on the transverse processes of the cervical 

 vertebrae, uniting anteriorly into one band which will readily be seen by looking 

 immediately ventral to the origin of the anterior part of the serratus ventralis. 

 Insertion, transverse processes of the cervical vertebrae; action, draw the ribs 

 forward and bend the neck. 



o) Intercostals. The intercostals are a set of muscles extending from one 

 rib to the next. The external layer is called the external intercostals and will be 

 seen in part by looking at the chest wall between the origins of the serratus 

 ventralis and dorsalis. Their fibers run obliquely backward and downward. 

 On cutting through some of them another layer, the internal intercostals, will be 

 seen inside of the external layer. The fibers of the internal intercostals run 

 obliquely forward and downward. Near the median ventral line the external 

 intercostals are lacking, so that the internal ones are exposed by cutting through 

 the scalenes. Action, external intercostals bring the ribs forward, internal 

 intercostals draw them back again. The intercostals are the chief respiratory 

 muscles of the thoracic wall. 



p) Epaxial muscles of the thorax. On cutting through the aponeurosis of the 

 serratus dorsalis, the thick mass of epaxial muscle is exposed. This may be 

 followed up into the neck by cutting through the splenius. In the thoracic 

 region the muscle is divisible into three parts of about equal width, a dorsal part 

 next the median dorsal line, the semispinalis dorsi; a median part, the 

 longissimus; and a ventral part, the iliocostalis. The latter is composed of a 

 number of separate bundles with prominent tendons between them, and lies on 

 the ribs to which it is attached. The longissimus may be traced forward to its 



