MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 39 



fused with the laterally lying m. semispinalis capitis with which it is in- 

 serted on the occipital squama between the splenius and the sub- 

 occipital muscles (fig. 15). 



M. extensor caudae lateralis: This is a long and strong muscle 

 extending from the lumbar region all the way to the end of the tail. 

 Its proximal fleshy belly changes distally into several tendons. In the 

 loins the muscle lies against the pedicles of the lumbar vertebrae, over 

 their costal processes, and is separated laterally by a very well-defined 

 space from the origins of longissimus on both sheets of the sacrospinal 

 aponeurosis. It then occupies the lateral sacral grooves, where it is 

 found between the abductors and the medial extensor of the tail. 

 Caudally, the subdivided belly and its tendons run along a trough 

 formed in the proximal vertebrae by the pedicles, the articular and the 

 costal processes; in the distal ones by the cranial articular processes, 

 the body and the very much modified costal and distal articular 

 processes. Origin is from (1) the anapophyses of the last four or five 

 lumbars, (2) the tubercles of the intermediate sacral crest, and (3) 

 the proximal articular processes of all caudal vertebrae. Insertion 

 takes place by 24 or 35 tendons individually attached to the proximal 

 articular processes of each of the caudal vertebrae, superficial to the 

 distal segments of the muscle. The first ends variably from Cd 3 to 

 Cd 5. The caudal aponeurosis provides individual fibrous covers for 

 the subdivisions of the belly and their respective tendons. 



M. abductor caudae medialis: This muscle is found only in the 

 sacral and proximocaudal regions, between and almost completely 

 covered by both the extensor and the abductor caudae lateralis. Its 

 fleshy fascicles originate from (1) the iliac tuberosity and the postero- 

 superior iliac spine, and (2) the dorsal sacroiliac ligaments. Insertion 

 is on the costal processes of the first four caudal vertebrae by an equal 

 number of tendons fastened to the cranial border of these structures. 

 This attachment is marked by a spine. 



M. abductor caudae lateralis: It has a segmented arrangement and 

 is larger than the medialis. Origin is from (1) the sciatic margin above 

 the greater sciatic notch and (2) the costal processes of caudal verte- 

 brae. A unit spans four segments proximaUy but is shorter toward the 

 end of the tail. Insertion is also on the costal processes. 



Sirena (1871) is the only one who has previously dealt to some 

 extent with the long muscles of the back in Alouatta, but his descrip- 

 tion of the different parts of the system is incomplete. This is probably 

 due to the great difficulties which one finds in following the details of 

 the several tendinous and fleshy attachments. He refers to the ilio- 

 costalis as m. sacro-lumbalis and adds that it lacks a m. cervicaHs 

 ascendens. Nishi (1919) has explained that what Arnold called m. 

 cervicalis ascendens in man is just the iliocostalis cervicis of other 



