VERTEBRATA. 



337 



fibres take various directions in accordance with the necessities of their 

 position. The muscles of the limbs are derived from the primitive muscle- 

 plates (p. 334) in Elasmobranchii, and probably in other orders of Fish as 

 also in Amphibia and Lacertilia. In higher Vertebrata they arise inde- 

 pendently, probably by an abbreviation of development. All the muscles 

 referred to above lie externally to the ribs and the transverse processes of 

 the vertebrae. They are hence termed episkeletal. A superficial portion 

 of this episkeletal system may retain its original independence of the 

 skeleton and forms the ' muscles of the skin.' It is sparingly represented 

 in Pisces, in Amphibia and Reptilia, except where moveable scales are 

 present, as in Gymnophiona and Ophidia. In Aves it is said to form the 

 erectors of the feathers, and in most Mammalia it is very well developed. 

 The Primates, however, retain but few traces of it (platysma myoides). In 

 Amniota there is a small system of hyposkeletal, or sub-vertebral muscles, 

 underlying the vertebral column, the origin of which is unknown. It is 

 also uncertain whether or not the fibro-muscular diaphragm of Mammalia 

 belongs to this last-named system. Both epi- and hypo-skeletal muscles are 

 striated. On the other hand, the musculature of the viscera with the ex- 

 ception of the heart, is, as a rule, non-striated. 



The skeleton, or endo-skeleton, assumes a vast importance in Verte- 

 brata. It consists of an axial and an appendicular portion. The former 

 includes the skull and backbone : the latter the skeleton of the paired 

 limbs and the girdles to which they are attached. 



The notochord, which forms the axis of the backbone, extends into 

 the skull as far forwards as the pituitary fossa. It undergoes atrophy, as a 

 rule, in the skull, even though it persists, more or less, in the backbone. 

 Exceptions are Cyclostomi and the Sturgeon (Acipenser). The skull itself 

 is derived from the following elements : (i) two parachordal cartilages 

 which lie one on each side of the anterior end of the notochord ; (2) two 

 trabeculae cranii underlying the fore-brain, and in shape resembling more 

 or less lyriform rods in contact near their anterior extremity, and diverging 

 posteriorly where they encircle the pituitary fossa and come into contact 

 with the two parachordals 1 with which they are continuous in development 

 in higher types (Bird, Pig), and with which they ultimately unite in all 

 classes. They fuse together anteriorly in the region of the nose except in 

 Eels. They retain in Ophidia the form of two rods in the region between 

 the nose and the parachordals. Except in Urodelous Amphibia and a few 

 other forms, the membranous floor of the pituitary region chondrifies con- 

 tinuously with the trabeculae, and is perforated only'by the carotid arteries. 

 Chondrification extends upwards from the parachordals and trabeculae in 

 the layer of indifferent cells investing the brain. The former constitutes 

 the occipital region. The trabeculae give origin to the basi- and ali- 



1 It is possible that the trabeculae are dissociated portions of the parachordals. 



Z 



