ACTION OF THE DIAPHRAGM. 



173 



(B) Expiration. 

 I. During Ordinary Respiration. 



The thoracic cavity is diminished by the weight of the chest, the elasticity of the 

 lungs, costal cartilages, and abdominal muscles. 



II. During Forced Expiration. 



The Abdominal Muscles. 



1. The abdominal muscles [including the obliquus externus and internus, and 

 transversalis abdominis] (JVn. abdominis internis anteriores e nervis inter costalibus, 

 8-12). 



2. Mm. intercostales interni, so far as they lie between the osseous parts of the 

 ribs, and the Mm. infracostales (JVn. intercostales). 



3. M. triangularis sterni (JVn. intercostales). 



4. M. serratus posticus inferior (Earn, externi nerv. dorsalium). 



5. M. quadratus lumborum (Ram. muscular e plexu lumbali). 



113. ACTION OF THE INDIVIDUAL EESPIRATORY MUSCLES. (A) Inspiration. (1) 

 The Diaphragm arises from the cartilages and the adjoining osseous parts of the lower six ribs 

 (costal portion), by two thick processes or crura, from the upper three or four lumbar vertebrae, 

 and a sternal portion from the back of the ensiform process. It represents an arched double 

 cupola or dome-shaped partition, directed towards the chest ; in the larger concavity on the 

 right side lies the liver, while the smaller arch on the left side is occupied by the spleen and 

 stomach. During the passive condition, these viscera are pressed against the under surface of 

 the diaphragm, by the elasticity of the abdominal 

 walls, and by the intra-abdominal pressure, so that 

 the arch of the diaphragm is pressed upwards into the 

 chest. The elastic traction of the lungs also aids in 

 producing this result. The greater part of the upper 

 surface of the central tendon of the diaphragm is 

 united to the pericardium. The part on which the 

 heart rests, and which is perforated by the inferior 

 vena cava (foramen quadrilaterum) is the deepest part 

 of the middle portion of the diaphragm during the 

 passive condition. 



Action of the Diaphragm. When the dia- 

 phragm contracts, both arched portions become 

 flatter, and the chest is thereby elongated from 

 above downwards. In this act, the lateral 

 muscular parts of the diaphragm pass from an 

 arched condition into a flatter form (fig. 138), 

 and during a forced inspiration the lowest 

 lateral portions, which during rest are in con- 

 tact with the chest-wall, become separated 

 from it. The middle of the central tendon 

 where the heart rests (fixed by means of the 

 pericardium and inferior vena cava) takes no 

 share in this movement, especially in ordinary 

 quiet breathing, but during the deepest in- Sagittal section through the second rib on 



the right side. When the arched mus- 

 cular part of the diaphragm contracts, a 

 wedge-shaped space, with its apex down- 

 wards, is formed around the circumfer- 

 ence of the lower part of the chest. 



spiration it sinks somewhat. 



Undoubtedly, the diaphragm is the most powerful 

 agent in increasing the cavity of the chest. Briicke 

 believes that in addition to increasing the length of 

 the thoracic cavity from above downwards, it also 

 increases the transverse diameter of the lower part of the chest. It presses upon the abdominal 

 viscera from above, and strives to press these outwards, thus tending to push out the adjoining 

 thoracic wall. If the contents of the abdomen are removed from a living animal, every time the 

 diaphragm contracts the ribs are drawn inwards. This, of course, hinders the chest from 

 becoming wider below, hence the presence of the abdominal viscera seems to be necessary for the 

 normal activity of the diaphragm. Every contraction of the diaphragm, by increasing the 



