118 



ciregory: notharctus, an American eocene primate 



heavy tails adapted for other purposes. From these plates (Fig. 29) may be derived the following gen- 

 eral conception of the musculature and mechanism of the tail in mammals: 



1. Extensor caudce medialis. — The deepest layer of muscles, on the dorsal aspect of the tail, consists 

 of the superior interspinal muscles (T, " interepineux superieurs" or " lumbo-sacro-coccygien " = extensor 

 caudse medialis, Reighard and Jennings) which lie around the dorsal spines and are serially homologous 

 with the spinalis dorsi and multifidus spinse. They are continued forward over the sacrum, on to the 

 lumbar region, and extend back some distance behind the coccyx, becoming gradually weaker in the 

 posterior tail vertebrae. Each segment of this series rises from the sides of the neural spine and from 

 the metapophysis, or prominence above the prezygapophysis ; it runs over the next following vertebra 

 and is inserted around the base of the neural spine of the second vertebra following. This muscle assists 

 in raising the tail. 



2. Extensor caudce lateralis. The superior sacro-coccygeal '))iuscles (R, " sacro-coccygien superieurs" 

 or " lumbo-sus-caudien " ) . — This series extends over the lumbar, sacral, and caudal vertebras, becoming 

 reduced posteriorly. Each segment rises from the area lying between the base of the prezygapophysis 

 and the transverse process; it is thus lateral to and more superficial than the superior interspinal muscles. 

 Each segment of this series ends in a long thin tendon, and these tendons form a large longitudinal bundle 

 running along the upper side of the tail; they are inserted into or near the posterior ends of the trans- 

 verse processes along with the intertransversal muscles (Z). The superior sacro-coccygeal muscles (R) 

 together with the superior interspinals (T) cooperate in raising the tail, and are hence named by many 

 authors levator cauda? internus and levator caudae externus, respectively. 



3. Abductor caudce internus. The intertransversals (Z, "intertransversaires"). — These deep- 

 seated muscles form a wedge-shaped series arising from the sides of the posterior sacral, coccygeal and 

 caudal vertebrte, inmiediately above and below the transverse processes and ventral to the superior 

 sacro-coccygeal muscles. Each one passes backward and is inserted into the posterior swelling of the 

 transverse process of the next following vertebra. They are the chief agents in curving the tail from 

 side to side. 



4. Inferior interspincd series (V, "sous-caudien, interepineux inferieurs"). — These arise from the 

 ventral surfaces of the centra and from the transverse processes of the sacral, coccygeal and caudal verte- 

 brae, and each segment is inserted posteriorly into the chevron of the second following vertebra?. They 

 assist in flexing the tail. 



5. Ischio-coccygecd (Q, " ischio-coccygien externe" abductor caudae ventralis). — This ischio-coccy- 

 geal muscle arises from the dorsal rim of the ilium above and behind the acetabulum; it passes upward 

 and backwartl and is inserted into the transverse processes of the coccygeal and first few caudal vertebrae. 



G. Ilio-coccygeal (U, "ileo-sous-caudien," part of levator ani group). — Arises from the ischial 

 spine above and behind the acetabulum; it passes upward and backward and is inserted into the ventral 

 surface of the upper caudal \-ertebrie. These muscles (Q, U) are broadest opposite the coccygeal and 

 upper caudal vertebrae, which have the largest chevrons, and diminish rapidly in the posterior caudal 

 vertebrae. They act powerfully to lower the proximal end of the tail; they are immensely developed 

 in the kangaroo and of moderate size in the lemurs. 



7. Injerior sacro-coccygeal series (R' "sacro-coccygien inftrieur) or "lumbo-sous-caudien^"). — 

 This is the chief antagonist of the superior sacro-coccygeal series; it arises from the inferior surfaces 

 of the sacrum and of the transverse processes of the caudal vertebrae. The fleshy belly of each segment 

 is continued posteriorly into a long tendon and these tendons together form a prominent bundle on the 



