828 COMPARATIVE ANATOMY 



ing the mesenchyme. The other three give rise to the "eye muscles" 

 which move the eye ball. (Fig. 466.) In general, 1, which lies in front 

 of the mouth, gives rise to four muscles, the inferior oblique and three 

 of the rectus muscles; 2, which lies in the region of the jaws, forms the 

 superior oblique muscles; while 3, in the hyoid region, develops the 

 lateral (external) rectus (in some animals also a retractor bulbi). The 

 origin of these muscles explains the distribution of the eye-muscle- 

 nerves, as each nerve supplies only the derivatives of a single myotome. 

 Several of the other myotomes disappear in development, but the 

 posterior becomes the so-called hypoglossal musculature. 



We have been describing only the origin of the contractile tissue of 

 the muscles. There is also a connective tissue to be considered. 



Mesenchyme cells invade the muscle fibres to form envelopes 

 (perimysium) which bind the fibres into bundles (fasciculi), these in 

 turn, are united by other envelopes called fascia. These connective- 

 tissue envelopes continue beyond where the contractile tissue leaves 

 off to form the cords or tendons by which the muscle is attached. The 

 more fixed point of attachment is called the origin, the less fixed the 

 insertion. Tendons may be of any shape ; such as long and slender, so as 

 to allow the muscle to lie in or near the trunk, the part to be moved being 

 in the appendage; or they may form broad flat sheets (aponeuroses). 

 These latter may occur not only at the ends but in the middle of a 

 muscle. Sometimes parts of tendons ossify, as in the patella or in the 

 "drum-stick" of the turkey. Such small rounded ossifications of this 

 kind are called sesamoid bones. In a few cases, even, as for example, 

 around the eye and mouth of mammals, the parietal muscles are without 

 attachment. Here they form rings which are used to diminish the size 

 of an opening (sphincter muscles). 



Muscles vary considerably as to shape, size, number of "heads" or 

 points of origin, and numbers of contractile portions. 



Muscles are usually arranged in antagonistic groups so that any 

 given action may likewise be reversed. We thus have flexors to bend a 

 limb and extensors to straighten it; elevators to close the jaw, depressors 

 to open it, etc. 



It is rather difficult to trace exact homologies. The test usually con- 

 sidered best is to trace the nerve supply, for every muscle derived from a 

 given myotome is innervated by branches of the nerve which also 

 originally connected with that segment. A further test is the origin and 

 insertion. The action of a muscle is of little value in a test for homo- 

 logies. 



A difficulty in the drawing of conclusions from specimens before one, 

 comes from the fact that a muscle may split into various layers either 

 longitudinally or transversely, and some even, though entirely different 

 in origin, may fuse together, while others, either in part or in whole, may 

 degenerate and disappear entirely. Should one take nerve supply as a 

 guide, as is usually done, it will be seen that the facial muscles, especially 



