44 U.S. NATIONAL MUSEUM BULLETIN 2 73 



Mm. interspinales : In the lumbar region a mixture of tendinous 

 and muscular fibers join the adjacent spinous processes. In the neck, 

 in relation with the underdevelopment of the spines of C 3, 4, 5 and 

 the enlargement of that of the axis, the interspinal muscles are ac- 

 cordingly modified. A pair of well-developed bellies, one at each side 

 of the midline, arises from the lateral surface of the spines of C 5 

 and 6. They pass upward to end on the flanges resulting from the 

 bifurcation of the axial spinous process. 



The four muscles involved in the formation of the suboccipital 

 triangle are regarded by Sirena (1871) as better developed, with rela- 

 tion to the region, than their equivalents in man. He describes paired 

 musculi intertransversarii in the cervical and lumbar regions only. 

 The interspinales of the neck are said to be like those of man and, in 

 the thoracic and lumbar regions, represented by tendinous fibers. 



Nerve supply of the short system. — The dorsal ramus of C I 

 passes through the postatlantal foramen and sends branches to all 

 four suboccipital muscles. The obliquus capitis inferior receives also 

 a branch from the dorsal ramus of C II. The intertransversarii and the 

 interspinales are supplied by branches of the corresponding dorsal 

 rami of the spinal nerves. 



Function of the epaxial muscles. — They are essentially exten- 

 sors of the head, neck, trunk, and tail when they contract bilaterally. 

 This role in the first two regions is particularly significant due to the 

 problems introduced here by the weight of the hyolaryngeal structures. 

 This large mass exerts a flexing influence upon the combined cranio- 

 cervical portion of the animal. Based on the electromyographic studies 

 of the back muscles in man (Morris, Benner, and Lucas, 1962), I think 

 that in Alouatta the components of the erector spinae act also as ipsi- 

 lateral rotators while the members of the deep oblique system are in- 

 volved in the contralateral action. The complex arrangement of the 

 fascicles of the long and deep oblique systems in the low thoracic and 

 lumbar regions of Alouatta suggests that the rotatory role of all these 

 muscles is not fully differentiated. 



The caudal extensions of the long and deep oblique systems have 

 attained a considerable degree of differentiation in accordance with 

 the prehensile abilities of the tail. This appendix is used for a wide 

 variety of purposes, from that of securing a hold on the branches while 

 moving, resting, or eating, to others such as manipulating the genitalia 

 (Carpenter, 1934). 



With respect to the long system, the origin of longissimus from the 

 metapophyses of the first lumbar and last two or three thoracic verte- 

 brae may perhaps be related to the locomotor habits of the animal. 

 The howling monkey is generally regarded as a sluggish creature, 

 acting more like an arboreal quadruped than as a brachiator. Erikson 



