474 THE MUSCULAK SYSTEM. 



Nerve-Supply. The intercostal muscles, levatores costarum, subcostal muscles, and traus- 

 versus thoracis, are all supplied by the anterior rami of the thoracic nerves. The diaphragm 

 receives its chief, if not its entire, motor supply from the phrenic nerves (C. 3. 4. 5.). It is 

 innervated also by the diaphragmatic plexus of the sympathetic, and is sometimes said to 

 receive fibres from the lower thoracic nerves. 



Actions. The act of respiration consists of two opposite movements inspiration and ex- 

 piration. 



1. The movement of expiration is performed by (1) the elasticity of the lungs, (2) the weight 

 of the chest walls, (3) the elevation of the diaphragm, (4) the action of muscles trans versus 

 thoracis and muscles of the abdominal wall. It is sometimes stated that the interosseous fibres of 

 the internal intercostal muscles are depressors of the ribs. 



2. The movement of inspiration results in the enlargement of the thoracic cavity in all 

 its diameters. Its antero-posterior and transverse diameters are increased by the elevation and 

 forward movement of the sternum, and by the elevation and eversion of the ribs, while its 

 vertical diameter is increased by the descent of the diaphragm. 



The muscles of inspiration are divided into two series ordinary and accessory. 



a. Ordinary Muscles. 



Diaphragm 



Intercostals 



Scaleni 



Serrati posteriores 



Levatores costarum 



Subcostales 



b. Extraordinary and Accessory Muscles. 



Quadratus lumborum 



Pectorales 



Serratus anterior 



Sterno-mastoid 



Latissimus dorsi 



Infra-hyoid muscles 



Extensors of the vertebral column 



Of the ordinary muscles the diaphragm is the most important. Its action is twofold 

 centrifugal, elevating the ribs and increasing the transverse and antero-posterior diameters of 

 the thorax, and centripetal, drawing downwards the central tendon and increasing the vertical 

 diameter of the thorax. Of the two movements the former is the more important. There has 

 been considerable diversity of opinion regarding the action of the intercostal muscles. It is 

 generally agreed that the external muscles elevate the ribs ; it is probable that the whole of each 

 internal muscle acts in the same way, although it has been stated by different observers that the 

 whole internal muscle is a depressor; or that the interosseous part is a depressor, the inter- 

 chondral portion of the muscle an elevator of the ribs. 



FASCIAE AND MUSCLES OF THE ABDOMINAL WALL. 



The space between the base of the bony thorax and the pelvis is filled up by 

 a series of muscular sheets, covered externally and internally by fasciae. 



FASCIAE. 



The fasciae of the abdominal wall are externally, the superficial and deep fasciae 

 internally, the fascia transversalis, which clothes the interior of the abdominal 

 cavity, and is continuous with the diaphragmatic, luinbo-dorsal, psoas, iliac, anc 

 pelvic fasciae, and is lined within by the subserous coat of extra-peritoneal tissue. 



The superficial fascia of the abdomen is liable to contain a large quantify 

 of fat. In the groin it is separated into two layers : a superficial fatty layer con 

 tinuous over the inguinal ligament with the fascia of the anterior surface o 

 the thigh (p. 402), and a deeper membranous layer attached to the medial half o 

 the inguinal ligament, and more laterally to the fascia lata of the thigh distal to tib 

 inguinal ligament. The two layers are separated by the lymph glands and th 

 superficial vessels of the groin. Higher up in the abdominal wall the two layer 

 blend together. As they pass downwards over the spermatic funiculus, they unit 

 to form the fascia and dartos muscle of the scrotum. The attachment of th 

 fascia to the groin prevents the passage into the thigh of fluid extravasated in th 

 abdominal wall. 



The deep fascia of the abdominal wall resembles similar fasciae in other situf 

 tions. It forms an investment for the obliquus externus muscle, and becomes thi 

 and almost imperceptible in relation to the aponeurosis of that muscle. 



