FASCIM 



371 



responding musculature, although variously modified in different forms, is found tliroughout the 

 vertebrate series. In the lower forms it seems to be differentiated directly from the segmental 

 trunk musculature and secondarily attached to the shoulder girdle, like the superficial and deep 

 musculature of the shoulder girdle previously described. In man, however, the muscle mass 

 from which these muscles arise is at all times in intimate union with the skeleton of the upper 

 limb, and the nerves which supply it are in much more intimate union with the brachial plexus 

 than are those of the shoulder-girdle muscles. For these reasons the three muscles are classed 

 with the intrinsic muscles of the arm. They have no certain representatives in the lower limb, 

 although the clavicular portion of the pectoralis major is considered by some to represent certain 

 adductor muscles of the thigh. Possibly they correspond in their embryonic origin with the 

 obturator internus group of the lower limb. 



In many of the mammals a subcutaneous muscle arises from the pectoral muscle mass and 

 extends over the axilla and the trunk. In man this musculature is frequently represented by 

 abnormal sUps of muscles, of which the 'axillary arch' and possibly the 'sternalis' are representa- 

 tives. A list of some of the abnormal muscles which are innervated from the anterior thoracic 

 nerves and are evidently derivatives of the pectoral muscle mass is given at the end of this section 



Fig. 359. — (After Eisler). Fascia of the Axillary Fossa. 



FASCIA 



In the tela subcutanea of the pectoral region the mammary gland is embedded between 

 two layers which ensheath the gland and are connected by dense fibre-bands. To a greater 

 or less extent the platysma extends into the tela of this region from above the clavicle. 



Muscle fasciae. — The pectoralis major is invested with a thin, adherent membrane, fascia 

 pectoralis, attached to the clavicle and the sternum and continued into the fascial invest- 

 ment of the external obhque, the serratus anterior (magnus), and the deltoid muscles, 

 and in to the axillary fascia. More important is the coraco-clavicular (costo-coracoid) fascia 

 fig. 358. This arises from two fascial sheets which invest the subclavius muscle and are at- 

 tached to the clavicle. From the inferior margin of this muscle the membrane is continued 

 to the superior margin of the pectoralis minor. Between the coracoid process and the first 

 costal cartilage it is strengthened to form the costo-coracoid ligament. Between this and the 

 pectorahs minor it is thin. At the superior margin of this muscle it again divides to form two 

 adherent fascial sheets, which, at the axillary margin of the muscle, once more unite to form a 

 firm membrane continued into the fascial investment of the coraco-brachialis and short head of 

 the biceps and into the axillary fascia. Above, dorsally, the membrane is adherent to the sheath 

 of the axillary vessels and nerves. 



Axillary fascia (fig. 359). — The arm-pit, or axillary fossa, is a pyramidal space bounded 

 by the pectoralis major and minor and coraco-brachialis muscles in front; by the latissimus 



