45 



CHAPTER II. 



THE MUSCLES AND THE VISCERA — A GENERAL VIEW OF 

 THE ORGANIZATION OF THE VERTEBRATA. 



The muscular system of the Vertebrata consists of muscles 

 related partly to the exoskeleton, partly to the endoskeleton, 

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

 unstriated muscular fibre. The latter is confined to the 

 vessels, the viscera, and the integument ; the parts of the 

 endoskeleton being moved upon one another exclusively by 

 sti'iated muscular 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 their 

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

 takes its origin in the protovertebrse ; each protovertebra 

 becoming differentiated, as we have seen, into three parts ; 

 a spinal ganglion and a segment of the vertebral endo- 

 skeleton, in the same plane, and a more superficial sheet 

 of muscular fibres. These muscular fibres are consequently 

 situated above the endoskeleton, or are episkeletal. Other 

 muscular fibres are developed below the endoskeleton, and 

 may be termed hyposkeletal muscles. The hyposkeletal 

 muscles are separated from the episkeletal, not only by the 

 endoskeleton of the trunk (or the vertebrae and their pro- 

 longations, the ribs), but by the ventral branches of the 

 spinal nerves. 



As the episkeletal muscles are developed out of the proto- 

 vertebrse, they necessarily, at first, present as many seg- 



