98 THE STRUCTURE OF MAN 



is necessary, to facilitate judgment and sharpen observation, in 

 dealing with both progressive and retrogressive variations, which 

 latter are the preliminary stages in degeneration. In the critical 

 examination of the muscles, as pointed out by Fiirbringer and 

 Eucre, it is primarily important to ascertain their innervation. 

 The nerve-supply is the safest criterion as to the morphological 

 value of a muscle. 



RETROGRESSIVE MUSCLES 



OF THE TKUNK 



The dorsal upper and lower serratus are, as is well known, 

 connected together by a strong silvery aponeurosis. This is 

 occasionally replaced by muscular tissue, which, in connection 

 with the upper serratus less frequently with the lower 

 may extend down as far as the sixth rib. This clearly points 

 back to a primitive condition in which the two muscles were 

 continuous. In contrast to this variation there occur others in 

 which the two serrati are much less developed than usual, so much 

 so that one or both of them may be entirely wanting. This is 

 very important, as it leads to the conclusion that the serrati, like 

 many other muscles, are being gradually transformed into tendinous 

 tissue. The cause of this must be sought in the modification of 

 the respiratory mechanism of the thorax, and the same would 

 appear to be the rationale of the many variations of these same 

 muscles observed in the Anthropoids (cf. ante, p. 45). 



The degeneration of the caudal region in the human body 

 has naturally been accompanied by a corresponding reduction of 

 the related muscles, i.e. especially of those the homologues of 

 which, in caudate Mammals, are strongly developed for moving 

 the tail. These are serial with the musculature of the trunk, 

 and can be divided into a ventral and a dorsal group. 'To the 

 latter belong the extensor and levator coccygis, which lie along 

 the posterior surface of the coccygeal vertebrae. This extra- 

 ordinarily thin muscle bundle arises either from the great sacro- 

 sciatic ligament or from the lowest end of the sacrum, and sends 

 out tendinous rays towards the apex of the coccyx. 



To the ventral series belongs the coccygeus muscle, which 

 arises from the spine of the ischium, runs along the lesser sacro- 

 sciatic ligament, and is inserted into the lateral edge of the coccyx. 

 This muscle brings about the lateral movement (abduction) of 



