310 THE MUSCLES. 



Action.— The diaphragm, in contracting, tends to become an inclined plane ; 

 its central portion is carried backwards, and the antero-posterior diameter of the 

 chest is increased. It is, therefore, essentially an inspiratory muscle. It may 

 also raise the ribs by making the mass of abdominal viscera its fixed point ; 

 it then acts as a reflected muscle, to which these viscera serve as a pulley. (The 

 diaphragm also aids the other abdominal muscles in expulsive efforts, and, when 

 affected with iiTegular spasmodic contractions, produces the peculiar phenomenon 

 in Man and some of the lower animals, known as hiccough.) 



Differential Characters of the Diaphragm in the other Animals. 



In the Ox, the pillars of the diaphragm are very lon<:]; and voluminous. " The attachments 

 of the muscular portion are much farther distant from the cartilaginous circle than in the 

 Horse, particularly at the superior part ; this disposition explains the innocuousness of puncture 

 of the paunch in the middle of the last intercostal space ; for in the Horse, when the instru- 

 ment is passed through this part, it penetrates the thorax " (communicated to M. Lecoq by 

 M. Tahourin). 



In the Sheep, we have not remarked that the costal attachments were more forward than 

 in the Horse. 



Rigot erroneously states that, in the Pig and Dog, the oesophagus passes between the two 

 pillars of the diaphragm ; on several occasions we have convinced ourselves that this tube 

 traverses the right pillar, as in tiie other animals. 



Comparison of the Diaphragm of Man with that of Animals. 



In the human diaphragm, the aponeurotic portion is divided into three leaflets, which has 

 caused it to be termed the aponeurotic trefoil. Between the middle and right leaflet is the 

 orifice through which the inferior vena cava passes. The openings for the passage of the 

 oesophagus and the aorta are situated between the two pillars. There are frequently met with, 

 in Man, one or two small accessory pillars, separated from the large pillars by an aperture that 

 afibrds a passage, on the right side, to the vena azygos and the sympathetic nerve, and on the 

 left side, to one of the lumbar veins and the other sympathetic nerve. 



The peripheric muscular portion always joins, posteriorly, the central muscular portion. 



Article II. — Muscles of the Anterior Limbs. 



These are divided into four principal groups : the muscles of the shoulder, 

 arm, forearm, and foot. 



Muscles of the Shoulder. 



These muscles are grouped around the scapula, and all act upon the arm, 

 which they extend, flex, abduct, adduct, etc. They form two regions : an 

 external or supra-scapular^ and an internal or subscapular . 



A. External Scapular Region. 



This comprises four muscles : the long abductor of the arm {teres externus), 

 the short abductor (postea spinatus minor, or teres mitior), the supra-spinatus, and 

 infraspinatus. These muscles are applied to the external surface of the scapula, 

 aud are covered by an aponeurotic layer. 



Preparation of the external scapular region. — Separate the limb from the trunk ; remove the 

 trapezius and mastoido-humeralis, to expose the external surface of the aponeurosis; take 

 away, also, the small pectoral muscle, after studying its mode of insertion into this fascia. 



This being accomplished, next remove the latter muscle to show the supra-spinatus, the 

 infra-spinatus, and long abductor muscle of the arm ; leaving only the strip which attaches the 

 anterior portion of the latter to the tuberosity of the scapular spine. 



To study the short abductor muscle, it is only necessary to cut the long abductor and supr» 



