194 HISTORY OF THE HUMAN BODY 



history of these elements as they gradually assume control of 

 such different parts as the jaws, the auditory ossicles and the 

 laryngeal cartilages ; such would be the history of the muscu- 

 lar system as it may sometime be written, a history even the 

 outlines of which are in many places still waiting to be es- 

 tablished. 



A great barrier in the way of morphological study of the 

 muscles lies in the fact that there is no definite criterion of 

 homology, no simple way of absolutely proving that a given 

 muscle in a certain animal is morphologically the same as a 

 similarly related one in another species. This can be proven 

 in a fairly satisfactory way by tracing the race history through 

 a series of forms, provided that no extensive hiatus occurs in 

 the series; but more often the animals to be considered are 

 isolated forms, separated by wide gaps from their nearest liv- 

 ing allies. With other organs the embryological record fur- 

 nishes valuable clews and often traces a continuous history, by 

 the aid of which the condition in isolated adult forms may be 

 interpreted ; but here not only are the embryological conditions 

 extremely difficult to interpret, but in the majority of cases 

 the historic stages are not there, and the final condition is seen 

 to arise suddenly from a mass of apparently undifferentiated 

 cells. 



In attempting to establish a basis for homology it is to be 

 remembered that a muscle is not always a definite organ like a 

 bone or blood vessel, but is rather a mass of muscular fibers set 

 apart for a more or less distinct purpose and differentiated 

 from the surrounding muscle masses in proportion to the defi- 

 mteness and precision of its action. One muscle may be en- 

 tirely isolated from the adjacent fibers by a firm cover of con- 

 nective tissue and provided with a special tendon attached to 

 a definite skeletal process; another may be a bundle of fibers 

 but partially separated from a larger mass and acting only in 

 connection with it. Often, too, the action produced by a mus- 

 cle is not precise enough to prevent numerous variations, and 

 thus may vary considerably in different individuals of a single 

 species or even in the two sides of the same individual. It is 



