444 THE MUSCLES AND FASCIA. 



costarum assist the External intercostals in raising the ribs. The Triangularis 

 sterni draws down the costal cartilages ; it is therefore an expiratory muscle. 



Muscles of Inspiration and Expiration. The muscles which assist the action 

 of the Diaphragm in ordinary tranquil inspiration are the Intercostals and the 

 Levatores costarum, as above stated, and the Scaleni. When the need for more 

 forcible action exists, the shoulders and the base of the scapula are fixed, and then 

 the powerful muscles of forced inspiration come into play ; the chief of these are 

 the Trapezius, the Pectoralis minor, the Serratus posticus superior and inferior, 

 and the Rhomboidei. The lower fibres of the Serratus magnus may possibly assist 

 slightly in dilating the chest by raising and everting the ribs. The Sterno- 

 mastoid also, when the head is fixed, assists in forced inspiration by drawing up 

 the sternum and by fixing the clavicle, and thus affording a fixed point for the 

 action of the muscles of the chest. The Ilio-costalis and Quadratus lumborurn 

 assist in forced inspiration by fixing the last rib (see page 458). 



The ordinary action of expiration is hardly effected by muscular force, but 

 results from a return of the walls of the thorax to a condition of rest, owing to 

 their own elasticity and to that of the lungs. Forced expiratory actions are 

 performed mainly by the flat muscles (Obliqui and Transversalis) of the abdomen, 

 assisted also by the Rectus. Other muscles of forced expiration are possibly the 

 Internal intercostals and Triangularis sterni (as above mentioned), and the Ilio- 

 costalis. 



THE DIAPHRAGMATIC REGION. 



Diaphragm. 



The Diaphragm (did(ppa-yfj.a, a partition wall ) (Fig. 290) is a thin musculo- 

 fibrous septum, placed obliquely at the junction of the upper with the middle third 

 of the trunk, and separating the thorax from the abdomen, forming the floor of the 

 former cavity and the roof of the latter. It is elliptical, its longest diameter being 

 from side to side, somewhat fan-shaped, the broad elliptical portion being hori- 

 zontal, the narrow part, which represents the handle of the fan, vertical, and 

 joined at right angles to the former. It is from this circumstance that some 

 anatomists describe it as consisting of two portions, the upper or great muscle of 

 the Diaphragm, and the lower or lesser muscle. It arises from the whole of the 

 internal circumference of the thorax ; being attached, in front, by fleshy fibres to 

 the ensiform cartilage ; on either side, to the inner surface of the cartilages and 

 bony portions of the six or seven inferior ribs, interdigitating with the Transver- 

 salis ; and behind, to two aponeurotic arches, named the ligamentum arcuatum 

 externum et internum, and to the lumbar vertebne. The fibres from these sources 

 vary in length ; those arising from the ensiform appendix are very short and 

 occasionally aponeurotic ; those from the ligamenta arcuata, and more especially 

 those from the cartilages of the ribs at the side of the chest, are longer, describe 

 well-marked curves as they ascend, and finally converge to be inserted into the 

 circumference of the central tendon. Between the sides of the muscular slip from 

 the ensiform appendix and the cartilages of the adjoining ribs the fibres of the 

 Diaphragm are deficient, the interval being filled by areolar tissue, covered on the 

 thoracic side by the pleurae ; on the abdominal, by the peritoneum. This is, 

 consequently, a weak point, and a portion of the contents of the abdomen may 

 protrude into the chest, forming phrenic or diaphragmatic hernia, or a collection 

 of pus in the mediastinum may descend through it, so as to point at the 

 epigastrium. 



The ligamentum arcuatum internum is a tendinous arch, thrown across the 

 upper part of the Psoas magnus muscle, on each side of the spine. It is connected, 

 by one end, to the outer side of the body of the first lumbar vertebra, being 

 continuous with the outer side of the tendon of the corresponding crus ; and, by 



nal intercostals are muscles of expiration, as they pulled the ribs down, while the Internal intercostals 

 pulled the ribs up and are muscles of inspiration ( Trans. CM. Phys. Philadelphia, Third Series, vol. i., 

 1875, p. 97). 



