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 Firbringer and 
Ruge, 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 TRUNK 
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, ve. 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 he along 
the posterior surface of the coccygeal vertebre. 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 
