MUSCULAR SYSTEM. 129 



a constituent part of all the active organs of vegetative life 5 

 so that the consideration of many parts of this system is 

 inseparable from the history of particular organs. The 

 contractity of the muscular fibre is intimately connected 

 with the extent of respiration in animals, being feeble in the 

 radiated and molluscous tribes, where the respiration is 

 small, and more energetic in the articulated classes, where 

 the respiration is greater, and generally aerial, and for the 

 same reason it is less powerful in the imperfectly aerated 

 reptiles than in the feathered tribes where the air permeates 

 every region of the body. It is by means of this system 

 that animals are enabled to move to and fro, to seize and 

 masticate their food, or to convey it through their alimen- 

 tary cavity, to circulate their fluids through the body, to 

 force all discharges from their system, to bring the surround- 

 ing element into contact with their blood, to produce various 

 sounds for mutual intercourse, and to vary to infinity the 

 outward form and expression of their body ; the different 

 parts of this great and complicated system can therefore be 

 classified and arranged according to the functions to which 

 they are most subservient. The movements of voluntary 

 muscles appear to be accompanied with a distinct perception 

 of their active state, and to become associated with feelings 

 of the mind ; so that they chiefly belong to the organs of 

 relation, or the functions of animal life, while those of the 

 involuntary muscles, unaccompanied by sensations, and more 

 independent of the rest of the body, are chiefly connected 

 with the organs and functions of vegetative life. In most 

 muscles the component fibres run parallel to each other, with 

 numerous minute nervous fibrils and capillary blood-vessels 

 running in the same longitudinal direction between them, 

 and freely anastomosing to produce a complicated net-work 

 throughout the whole texture. Although the muscular fibres 

 are organs distinct in their whole course, they are united 

 into fasciculi, and these into entire muscles by means of cellu- 

 lar tissue, and they are generally inserted into strong, tough, 

 fibrous tendons, which are less supplied with nerves and 

 blood-vessels than the muscles themselves, and are destitute 

 of irritability. The muscles of the highest classes of red- 

 blooded animals, during their development, pass through 

 the soft, colourless, homogeneous, and gelatinous condition 



PART I. K 



