DELTOIDEUS 365 



The deltoid in many of the mammals and the lower vertebrates is represented by separate 

 scapulo-humeral and cleido-humeral portions. The cleido-mastoid in some mammals is con- 

 tinued into the deltoid. The teres minor, which is innervated by the same nerve, may be looked 

 upon as a derivative of the deltoid, although in man it is anatomically more intimately connected 

 with the infraspinatus. The teres major may be looked upon as a speciahsed portion of the 

 more primitive latissimus dorsi. The comparative anatomy of the shoulder muscles through- 

 out the vertebrate series is a somewhat intricate subject, owing to the great variations exhibited 

 in the form and attachment of the shoulder girdle. 



The muscles of this group show more or less marked resemblances to certain muscles of the 

 lower limb. The deltoid and the teres minor probably represent the tensor fascia? latse, the glu- 

 teal fascia, and the upper part of the gluteus maximus; the latissimus dorsi and teres major, 

 the lower portion of the gluteus maximus; and the subscapularis, the gluteus medius and mini- 

 mus, and the piriformis. The subscapular and axillary nerves, which supply the arm muscles 

 mentioned, therefore represent in the main the nerves to the gluteal muscles, and the gluteal 

 branch of the posterior cutaneous nerve of the thigh. The infraspinatus muscle probably 

 represents the iliacus; the supraspinatus possibly the pectineus muscle of the lower limb. 



FASCItE 

 (Figs. 351, 357, 359, 362) 



The tela subcutanea covering the regions occupied by these muscles contains considerable 

 fat. In most regions it is not readily separable into two distinct layers. In the neighbourhood 

 of the shoulder-joint it is adherent to the underlying musculature and the axiUary fasciae. Over 

 the acromion there is a well-marked subcutaneous bursa, bursa subcutanea acromialis. 



Muscle fasciae. — The deltoid and latissimus dorsi muscles are throughout the greater part 

 of their extent superficially placed. They are covered by an adherent fascial layer, which, 

 above, is attached to the clavicle and to the spine of the scapula. VentraUy it is continued over 

 and fuses with the fascia covering the pectorahs major, serratus anterior, and external oblique 

 muscles. On the back it extends as a thin sheet between the dorsal margin of the deltoid and 

 the upper margin of the latissimus dorsi, and is continued dorsally into the fascial investment 

 of the rhomboid muscles. The lateral fascial extension of the trapezius becomes fused to the 

 dorsal surface of this sheet. Toward the armpit the fascial investment of the deltoid and latiss- 

 imus dorsi muscles is continued into the axillary fascia, and on the back of the arm it is con- 

 tinued into the fascial investment of the triceps. 



The supraspinatus muscle lies beneath the trapezius. It is covered by a dense adherent 

 fascial layer which is separated from the trapezius by loose connective tissue which usually 

 contains a considerable amount of fat. 



The infraspinatus and the two teres muscles lie beneath the musculo-fascial layer composed 

 of the deltoid, the latissimus dorsi, and the fascial sheet described above. Each of the three 

 muscles has a special fascial investment which is bound to the scapula about the region of attach- 

 ment of the muscle to the bone. Where two of the muscles adjoin, their fascia? gives rise to 

 intermuscular septa. Septa of this nature are found between the infraspinatus and each of the 

 teres muscles, and between the teres minor and the teres major. The intermuscular septum 

 between the infraspinatus and teres minor muscles is often incomplete. The fascia covering 

 the teres major is so delicate as hardly to deserve the name, except near the origin of the muscle. 

 Near the spine the fascia covering the deep surface of the deltoid is often fused to that covering 

 the infraspinatus. 



The subscapularis muscle is invested by a moderately dense fascia which is bound to the 

 scapula along the periphery of the attachment of the muscle. For a short distance this fascia 

 is fused with the fascial investment of the teres major near the origin of the latter muscle, so that 

 an intermuscular septum is formed. From the ventro-lateral margin of the fascia covering the 

 subscapularis muscle a sheet of fascia is continued below the axillary fascia into the fascia cover- 

 ing the serratus anterior (magnus). 



MUSCLES 



The deltoideus (figs. 355, 356, 360). — Origin. — Fleshy from the lateral border and upper 

 surface of the acromion and from the ventral border and upper surface of the lateral third of the 

 clavicle, and tendinous from the spine of the scapula. Some fibre-bundles also at times arise 

 from the deep fascia of the muscle where it overlies and is fused to the fascia of the infraspinatus 

 muscle near the spine. 



Insertion.-— Into the deltoid tuberosity of the humerus by a strong tendon arising from 

 numerous tendinous bands within the muscle (fig. 364). 



Structure. — In structure the deltoid muscle is complex. Three portions may be recognised: 

 — a clavicular, an acromial, and a spinous. The first and last are composed of long fibre-bundles 

 which take a slightly converging course and are inserted by aponeurotic tendons respectively 

 on the front and back of the V-shaped area of insertion of the muscle. The acromial portion, 

 on the other hand, is multipenniform in composition. Four or five tendinous expansions descend 

 into the muscle from the acromion, and three up into the muscle from the tendon of insertion. 

 From the acromion and from the descending tendinous processes fibre-bundles run to be inserted 

 on the sides of the ascending processes and into the tendons of insertion of the clavicular and 

 spinous portions of the muscle. 



Nerve-supply. — The axillary (circumflex) nerve passes across the costal surface of the 

 muscle near the tendon of insertion and gives off rami which enter lateral to the middle of the 

 muscle. The nerve fibres are derived from the (fourth), fifth, and sixth cervical nerves. 



Action. — When the whole muscle contracts, the arm is abducted (raised laterally) to a 



