146 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



serratus ven trails. They lie internal to the sternomastoid, cleidomastoid, etc., 

 previously identified and farther posteriorly they form the layer next internal 

 to the pectoralis muscles. Action, raise the ribs and bend the neck. 



0) Serratus dor sails (posterior). The dorsal half of the thorax underneath 

 the latissimus dorsi, trapezius, and rhomboideus muscles is covered by a strong 

 aponeurosis (part of the lumbodorsal fascia), in the ventral part of which muscle 

 fibers are present which are inserted on the ribs by slips. The foremost slips 

 are quite fleshy and take their origin by a tendon from the median dorsal 

 line of the neck. This muscle is the serratus dorsalis. Action, raises the ribs 

 craniad. 



p) Intercostals. On the sides of the chest a series of muscles will be seen 

 running from one rib to the next one. They are the external intercostals. They 

 extend on the chest wall ventral to the insertion of the serratus dorsalis, which 

 muscle in fact covers their most dorsal portions. Origin, posterior margins of 

 the vertebral ribs; insertion, anterior margins of the succeeding vertebral ribs; 

 action, pull the ribs forward. Observe that the fibers of the external intercostals 

 are directed obliquely backward. On carefully cutting through any of the 

 external intercostals, a layer of internal intercostals will be found inside of them, 

 their fibers being directed obliquely forward. The internal intercostals are 

 best seen in the ventral thoracic wall, internal to the scalenes, which may be cut 

 through. Here between the costal cartilages the internal intercostals are not 

 covered by the external intercostals. Origin and insertion, margins of the 

 vertebral and sternal ribs; action, lower the ribs. The intercostals are the chief 

 muscles concerned in the respiratory movements of the thorax. The scalenes, 

 serratus, and other muscles assist. 



q) Epaxial muscles of the thorax. The mass of epaxial muscles is conspicuous 

 running along the dorsal part of the thorax. This mass passes internal to the 

 serratus dorsalis which should be cut through; it lies upon the dorsal portions of the 

 ribs and thus conceals the dorsal portions of the intercostal muscles. It is covered 

 by the tough shining lumbodorsal fascia, which should be removed. The epaxial 

 mass is easily divisible into a narrow median portion next to the median dorsal 

 line, the semispinalis dorsi, and a very thick lateral portion, the longissimus. 

 The latter gives off on its ventral margin the narrow iliocostalis lying on the ribs 

 to which it is inserted. The longissimus is the continuation of the sacrospinalia 

 and is inserted on the ribs. The epaxial mass may be followed along the neck 

 by cutting the splenius. 



The attention of the student is directed to the fact that the intercostal muscles 

 represent the original layer of myotomes, all of the muscles external to them 

 having been derived by processes of budding and splitting. The serratus 

 muscles beautifully illustrate the method of origin of a muscle by buds from a 

 number of myotomes, since the slips by which they arose from the intercostals 

 are still present. 



