THORACIC-ABDOMINAL MUSCLES 423 



Expiration is produced not only by the part ofthe internal intercostals which lie between the 

 bony ribs, and by the abdominal muscles, but also by the lumbar iUo-costales and by the quad- 

 ratus lumborum. 



The intrinsic muscles of the thorax and abdomen are derived from the twelve 

 thoracic myotomes and the first one or two lumbar and are innervated by the 

 corresponding nerves, while the musculature of the shoulder girdle and arm which 

 covers the intrinsic muscles of the thorax is of cervical origin an d is innervated 

 by cervical nerves. The diaphragm is likewise of cervical origin and is innervated 

 by the phrenic nerve from the cervical plexus. 



The intrinsic muscles of the back extend over the thoracic musculature (ex- 

 ternal intercostals and levators of the ribs, fig. 383) and in turn are in part covered 

 by muscles which extend dorsally from the thoracic region (posterior serrate 

 muscles, fig. 380). 



The intrinsic thoracic-abdominal muscles are composed laterally of three layers 

 of sheet-like muscles. 



In the external layer the fibre bundles run downward and ventralward. This 

 layer is represented in the thoracic region by the external intercostal muscles, the 

 levators of the ribs and the posterior serrate muscles. The fibre-bundles of the 

 external intercostals (fig. 385), extend between each pair of ribs but between the 

 costal cartilages are replaced by fibrous tissue, the external intercostal ligaments. 

 The levatores costarum (fig. 383), extend from the transverse process of one 

 vertebra to the rib which articulates with the next vertebra below and in some 

 instances the fibre bundles are continued to the second rib below. 



The serratus posterior superior and inferior (fig. 380), are derivatives of the 

 external oblique which during development wander in part over the intrinsic 

 dorsal musculature. The superior serrate arises from the spines of the last two 

 cervical and first two thoracic vertebrae and is inserted into the second to the 

 fifth ribs. The inferior serrate muscle arises from the spines of the last two tho- 

 racic and first two lumbar spines and is inserted into the last four ribs. The fibre- 

 bundles of this muscles therefore take a direction opposite to that of the other 

 muscles of the group. The muscles aid in inspiration. In the abdominal region 

 the external layer is represented b}^ the external oblique muscle (fig. 387). This 

 arises bj^ digitations from the last seven ribs and is inserted into the crest of the 

 ilium and by means of a broad flat aponeurosis into the linea alba in the mid- 

 ventral line and into the inguinal ligament below. The external intercostal, 

 levatores costarum, and posterior serrate muscles are innervated from branches 

 which arise near the tubercles of the ribs. The external obhque muscles are inner- 

 vated by branches which in large part arise in conjunction with or from the lateral 

 branches of the anterior divisions of the last seven thoracic nerves and frequently 

 also by branches from the ilio-hj^pogastric. 



The middle layer of the lateral thoraco-abdominal musculature is composed of 

 fibre-bundles which run downward and backward obliquety across the fibre-bundles 

 of the external layer. In the thoracic region it is represented by the internal 

 intercostal and subcostal muscles. The internal intercostal (fig. 385) muscles lie 

 between the costal cartilages and between the ribs as far dorsalward as the angles, 

 beyond which they are replaced by membranous tissue and by the subcostal 

 muscles. The latter, instead of extending from one rib to the next rib below, ex- 

 tend to the second or third rib below. They are best developed in the lower part 

 of the thoracic cavity. In the abdominal region the middle layer is represented by 

 the internal oblique muscle (fig. 388). This arises from the lumbo-dorsal fascia, 

 the crest of the ilium and the inguinal ligament and is inserted into the sheath of 

 the rectus abdominis muscle and into the inferior margins of the ventral extremi- 

 ties of the three lower ribs. The aponeurosis, which helps to form the sheath of 

 the rectus, divides in the upper abdominal region into two layers, one of which 

 passes in front and the other of which passes behind the rectus to be inserted into 

 the linea alba in the mid-ventral line. In the lower third of the ventral abdominal 

 wall both layers pass in front of the rectus. The fibre-bundles which compose the 

 internal oblique muscles do not all follow the usual course of the fibre-bundles of 

 this layer. At the level of the iliac crest they pass nearty transversely across the 

 body and below here they slant downward and forward. Just above the in- 

 guinal ligament and medial to its centre the internal oblique muscle is continuous 

 with the thin cremaster muscle (fig. 389), which is prolonged over the spermatic 

 cord and the tunica vaginalis of the testis and epididymis in the male and over the 



