26 NATURAL HISTORY OF VERTEBRATES. 



phibians to show that the radius itself is not part of the principal axis, but a secondary 

 axis developed beside it in response, perhaps, to terrestrial locomotion. 



THE MUSCULAR SYSTEM. 



Muscular tissue is characterized by the property of contractility, which may be 

 evoked by a stimulus applied directly to the muscular fibres, or conveyed to them 

 through the channel of nerve fibres. In the latter case the stimulus may be trans- 

 mitted consciously by the will, or it may be entirely involuntary. A large projwrtion 

 of the muscular tissue which is not directly under the control of the will differs from 

 that which is, in being histologically simpler, and in replying less quickly to a stimu- 

 lus. A rough distinction of voluntary and involuntary muscles is thus made, the 

 latter being exemplified by the minute muscles in tlie skin which erect the hairs, etc., 

 and by the greater masses of tissue which effect the propulsion of the food along the 

 alimentary canal, and of the blood through the vessels. 



The muscles which we discuss under this heading, however, are those that are con- 

 nected directly or indirectly with skeletal parts, and which bring about change of 

 relative position of these. They constitute the flesh or musculature of the body, and 

 are of course chiefly employed in locomotion and ])rehension. It is oidy comparatively 

 recently that much attention has been given to this branch of anatomy — Myology, as 

 it is called — but it is obvious, from what has been said above, that the most intimate 

 connection exists between this study and the study of the bones to which the muscles 

 are attached, as well as of the motor nerves which end in the muscles. 



Reference has already been made to the fact that the muscular system partakes in 

 the segmentation of the body. This can be seen in its most unaltered condition in a 

 fish (Fig. 10), where the flesh is divided up into muscle-flakes or segments (myotomes) 

 sepai'ated by membranous partitions or septa (myocommata). When the latter are 

 dissolved by boiling, the flesh readily separates into the flakes, the sm-faces of which are 

 often much curved, so that several different flakes appear in the same frontal plane (Fig. 

 11). In addition to these flakes which constitute the musculature of the trunk, there 

 ai-e other muscles also segmentally arranged in connection with the unpaired fins, but 

 it is not so easy to detect a segmental arrangement in the muscles which perform the 

 more complicated movements of the jaws, the gill-skeleton, and the paired fins. 



■\Vith the change to terrestrial life, the paired limbs, taking on the function of the 

 support of the body, become more complicated in their skeletal parts, and conse- 

 quently also in the muscles destined for their movements. How multifarious these 

 movements are can only be realized by reflecting on the extremely different uses 

 to which the paired limbs are put, in prehension and locomotion. The various kinds 

 of the latter, whether terrestrial, arboreal, aerial, or aquatic, are accompanied by cor- 

 res])onding modifications of the musculature. It is possible, nevertheless, to detect 

 homologies by careful study of the nerve su]i])ly, as well as of the ]ioints of attach- 

 ment (origin and insertion) to the bones. Tiie latter feature is that which ought to 

 determine the nomenclature of muscles, although general terms expressing action, 

 such as abduction, adduction, flexion, and extension are convenient and in frequent use, 

 and are sometimes, indeed, modified so as to designate the aspect of a limb on which 

 particular groups of muscles are situated. Tiius the liiceps muscle occupies the 

 'flexor' surface of the arm. 



The only guide as to the origin of the liml)-muscles from particular segments of 



