292 MUSCLES AND FASCIAE. 



The Diaphragm is arched, being convex towards the chest, and concave to the 

 abdomen. The right portion forms a complete arch from before backwards, being 

 accurately moulded over the convex surface of the liver, and having resting upon 

 it the concave base of the right lung. The left portion is arched from before 

 backwards in a similar manner ; but the arch is narrower in front, being encroached 

 upon by the pericardium, and lower than the right, at its summit, by about three 

 quarters of an inch. It supports the base of the left lung, and covers the great 

 end of the stomach, the spleen, and left kidney. The central portion, which sup- 

 ports the heart, is higher, in front at the sternum, and behind at the vertebra, 

 than the lateral portions; but deeper, this is reversed. 



The height of the Diaphragm is constantly varying during respiration, being 

 carried upwards or downwards from the average level ; its height also varies 

 according to the degree of distension of the stomach and intestines, and the size 

 of the liver. After a forced expiration, the right arch is on a level, in front, with 

 the fourth costal cartilage ; at the side, with the fifth, sixth, and seventh ribs ; and 

 behind, with the eighth rib : the left arch being usually the breadth of from one to 

 two ribs below the level of the right one. In a forced inspiration, it descends 

 from one to two inches ; its slope would then be represented by a line drawn from 

 the ensiform cartilage towards the tenth rib. 



Nerves. The Diaphragm is supplied by the phrenic nerves. 



Actions. The action of the Diaphragm modifies considerably the size of the 

 chest, and the position of the thoracic and abdominal viscera. During a forced 

 inspiration, the cavity of the thorax is enlarged in the vertical direction from 

 two to three inches, partly from the ascent of the walls of the chest, partly from 

 the descent of the Diaphragm. The chest, consequently, encroaches upon the 

 abdomen ; the lungs are expanded, and lowered, in relation with the ribs, nearly 

 two inches; the heart being drawn down about an inch and a half, the descent 

 of the latter organ taking place indirectly through the medium of its connec- 

 tion with the lungs, as well as directly by means of the central tendon to which 

 the pericardium is attached. The abdominal viscera are also pushed down (the 

 liver, to the extent of nearly three inches), so that these organs are no longer 

 protected by the ribs. During expiration, when the Diaphragm is passive, it is 

 pushed up fey the action of the abdominal muscles ; the cavity of the abdomen, 

 with the organs contained in it, encroaches upon the chest, by which the lungs 

 and heart are compressed upwards, and the vertical diameter of the thoracic cavity 

 diminished. The Diaphragm is passive when raised or lowered by the abdominal 

 organs, independently of respiration, in proportion as they are large or small, full 

 or empty ; hence the oppression felt in the chest after a full meal, or from flatulent 

 distension of the stomach and intestines. 



In all expulsive acts, the Diaphragm is called into -action, to give additional 

 power to each expulsive effort. Thus, before sneezing, coughing, laughing, and 

 crying, before vomiting, previous to the expulsion of the urine and faeces, or of 

 the foetus from the womb, a deep inspiration takes place. 1 



MUSCLES OF THE 

 These muscles are described with the anatomy of the Perineum (p. 778). 



' For a detailed description of the general relations of the Diaphragm, and its action, refer to 

 Or. SIBSON'S "Medical Anatomy." 



