APONEUROS1S 361 



muscle fibres are inserted into the integument, as on the side of the neck, the 

 face, and around the margin of the anus. It is very dense in the scalp, in the 

 mamrnre, the palms of the hands and soles of the feet, forming a fibrofatty layer 

 which binds the integument firmly to the subjacent structure. 



The superficial fascia connects the skin to the subjacent parts, facilitates the 

 movement of the skin, serves as a soft medium for the passage of vessels and nerves 

 to the integument, and retains the warmth of the body, since the fat contained in 

 its areolse is a poor conductor of heat. 



The deep or aponeurotic fascia is a dense, inelastic, unyielding fibrous mem- 

 brane, forming sheaths for the muscles and affording them broad surfaces for 

 attachment. It consists of shining tendinous fibres, placed parallel' to one 

 another, and connected by other fibres disposed in a rectilinear manner. It is 

 usually exposed on the removal of the superficial fascia, forming a strong in- 

 vestment, which not only binds down collectively the muscles in each region, but 

 gives a separate sheath to each, as well as to the vessels and nerves. The fascia? 

 are thick in unprotected situations, as on the outer side of a limb, and thinner on 

 the inner side. The deep fasciie assist the muscles in their action by the degree 

 of tension and pressure they make upon their surface; and in certain situations 

 this is increased and regulated by muscular action; as, for instance, by the Tensor 

 fasciae femoris and Gluteus maximus in the thigh, by the Biceps in the upper 

 and lower extremities, and Palmaris longus in the hand. In the limbs the fascia? 

 not only invest the entire limb, but give off septa which separate the various 

 muscles, and are deeply attached to the periosteum; these prolongations of fasciae 

 are usually spoken of as inter muscular septa. 



Development of the Skeletal Musculature. The voluntary muscles are developed from 

 the myotomes of the mesodermal somites. Portions of the myotomes retain their position along 

 the body axis in segmental order, such as the short muscles of the vertebral column. Others 

 migrate into the body wall, where again they may retain their segmental relation (intercostal 

 muscles), or may fuse with adjacent segments to form flat muscles such as those in the abdom- 

 inal wall. In the limbs, portions of myotomes wander into the limb buds and there undergo 

 fusions and alterations in form to produce the limb muscles, thus losing their segmental char- 

 acter, but retaining the appropriate segmental nerve supplies. Some of the limb muscles expand 

 and migrate secondarily toward the dorsimesal line (e. cj., Trapexius, Latissimus) or toward 

 the ventromesal line (e. g., Pectoralis major). Again, muscles may migrate cephalad (e. g., 

 the facial muscles derived from the hyoid arch) or caudad (P. g., Serratus magnus). In all 

 cases the musc-les carry with them the segmental nerves of the myotomes from which they were 

 derived; the most striking example is the Diaphragm which is derived from the third, fourth, 

 and fifth cervical myotomes, and is supplied by the phrenic nerves, which spring from the 

 third, fourth, and fifth cervical nerves. Some of the derivatives of the myotomes degenerate 

 and are converted into aponeuroses (e. g., epicranial aponeurosis) or into ligaments (e. g., great 

 sacrosciatic ligament and external lateral ligament of the knee). 



Various disturbances of development characterize what are known as variable or anomalous 

 muscles. Such muscles may be wanting, or excessively developed in the way of accessory 

 portions or with atypic attachments. The resemblance which such muscles in the human subject 

 bear to certain muscles in lower animals has been regarded by some anatomists as an expression 

 of atavistic reversion. 1 



The muscles and fasciae may be arranged according to the general division 

 of the body, into those of the cranium, face, and neck; those of the trunk; those of 

 the upper extremity or pectoral limb; and those of the lower extremity or pelvic 

 limb. 



1 See, however, Huntington, American Journal of Anatomy, 1902-03, vol. ii, p, 157, 



