CHAPTER n. 



THE MUSCLES AND THE VISCBEA A GENEEAl YIEW OF THB 



ORGANIZATION OF THE TEETEBEATA. 



The muscular system of the Vertehrata consists of muscles 

 related partly to the exoskeleton, partly to the endoskeletoii, 

 and partly to the viscera, and formed both of striated and un- 

 striated muscular fibre. The latter is confined to the vessels, 

 the viscera, and the integument ; the parts of the endoskele- 

 ton being moved upon one another exclusively by striated mus- 

 cular fibre. The muscles of the endoskeleton may be divided, 

 like the endoskeleton itself, into one system appertaining to 

 the trunk and head, and another belonging to the limbs. 



The Muscular System of the Trunk and Head. — This con- 

 sists of two portions, which differ fundamentally in theii 

 origin, and in their relations to the endoskeleton. The one 

 takes its origin in the protovertebrse ; each protovertebra be- 

 coming differentiated, as we have seen, into three parts ; a 

 spinal ganglion and a segment of the vertebral endoskeleton, 

 in the same plane, and a more superficial sheet of muscular 

 fibres. These muscular fibres are consequently situated above 

 the endoskeleton, or arc episTceletal. Other muscular fibres are 

 developed below the endoskeleton, and may be termed hypo- 

 sJceletal muscles. The hyposJceletal muscles are separated from 

 the episkeletal, not only by the endoskeleton of the trunk (or 

 the vertebras and their prolongations, the ribs), but by the 

 ventral branches of the spinal nerves. 



As the episkeletal muscles are developed out of the proto- 

 yertebrEB, they necessarily, at first, present as many segments 

 as there are vertebras, the interspaces between them appearing 

 as intermuscular septa. The development of the hj'poskeletal 

 muscles has not been worked out, but it appears to take place 

 much later than that of the episkeletal set. 



