i THE MUSCLES OF THE ABDOMINAL WALL. 397 
muscular fibres on the left side). By its lateral margins the diaphragm is in 
contact with the thoracic wall beyond the reflection of the pleura, and behind with 
the cesophagus and descending thoracic aorta. The under surface of the diaphragm 
is concave, and is for the most part invested by peritoneum. It is in relation with 
the liver, stomach, spleen, kidneys, suprarenal bodies, duodenum and pancreas, the 
inferior vena cava, and the branches of the cceliac axis. Its vault is higher on the 
right side than on the left, owing to the upward projection of the liver on that side. 
The diaphragm is found as a complete septum between the thorax and abdomen only in 
mammals. It is occasionally deficient in the human subject, producing hernia of the diaphragm, 
either into the pericardial cavity through the central tendon, or into the thoracic cavity through 
the lateral portions of the muscle. A rare condition is congenital deficiency of a part of the 
lateral half of the muscle, generally placed posteriorly, and on the left side. This produces, by 
continuity of the peritoneum and pleura behind the diaphragm, a congenital diaphragmatic 
hernia. 
NERVE SUPPLY. 
The intercostal muscles, levatores costarum, infra-costal muscles, and triangularis sterni, are 
all supplied by the anterior primary divisions of the intercostal 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 said to receive fibres from the lower 
intercostal nerves. 
ACTIONS. r 
The act of respiration consists of two opposite movements, inspiration and expiration. 
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—triangularis 
sterni 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 (the elevation and forward movement of the sternum, and 
the elevation and eversion of the ribs) produces enlargement of the thoracic cavity antero- 
posteriorly and transversely. Its vertical Miotdetey is increased by the descent of the diaphragm. ' 
The muscles of inspiration are divided into two series—ordinary and accessory. 
Infra-hyoid muscles 
Extensors of the spine 
a. Ordinary Muscles. 6. Extraordinary and Accessory Muscles. 
| yr) — Ss oe | os 7 
| Diaphragm | Quadratus lumborum 
| Intercostals | Pectorales 
| Scaleni | Serratus magnus 
| Serrati postici | Sterno-mastoid 
| Levatores costarum | Latissimus dorsi 
| 
Of the ordinary muscles, the diaphragm is the must 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. 
FASCIA AND MUSCLES OF THE ABDOMINAL WALL. 
The space between the margins of the bony thorax and the pelvis is filled up by 
a series of muscles, covered externally and internally by fasciw. The muscles are 
in three series—anterior, lateral, and posterior. The anterior muscles are the 
pyramidalis and rectus abdominis; the lateral muscles are the obliqui, externus and 
internus, and the transversalis abdominis; the posterior muscles are the quadratus 
lumborum, and the psoas (magnus and parvus) and iliacus (described already). 
a 
