170 U.S. NATIONAL MUSEUM BULLETIN 2 73 



cervical interspinals and the corresjionding changes in the spinous 

 ])rocesses of the region are more clearly dependent on the needs for 

 keeping the neck extended. 



It should now be said that the epaxial muscles, as well as the 

 ventral ones, are highly organized in the tail of Alouatta; but i)robably 

 not to a greater degree than in other prehensile-tailed monkeys. 



It is interesting to note that the scalenus brevis posterior of the 

 lateral group of hypaxial muscles has a variable development in dif- 

 ferent specimens of howlers. Its location (fig. 17) allows it to act as an 

 additional extensor of the cervical spine and it appears as a robust 

 and well-developed column in some cases, but as an inconspicuous 

 longitudinal bundle in others. This variability indicates that the ])at- 

 tern of the muscle is as yet not definitely set. 



Of the muscles in the ventral group, the sternohyoideus, sterno- 

 thyroideus, thyrohyoideus, costothyroideus, and geniohyoideus are 

 considerably enlarged (fig. 3) in relation to Cebus and the Atelinae. 

 They are of importance in keeping the hyolaryngeal structures in 

 place and perhaps also in modifying the voice. Contraction of the 

 sternohyoideus might help to close the hyolaryngeal canal by pressing 

 the buUa against the epiglottis and also to empty the saccules by its 

 action against the thyroid lamina (see fig. 11 in Schon, 1964a). The 

 sternothyroideus and costothyroideus lower the thyroid cartilage. The 

 second muscle may, in addition, contribute to the inrush of air into 

 the sacs and canal by pulling apart the upper margins of the plates. 

 The thyrohyoideus is possibly an antagonist of the costothyroideus 

 in that its contraction shortens the distance between the cornu bran- 

 chiale I and the thyroid cartilage, therefore emptying the air sacs. 

 The functional significance of the geniohyoideus was discussed else- 

 where (Schon, 1964a). The subvertebral and perineal groups do not 

 offer significant differences from those of Ateles, Cebus, and Brachyteles 

 (see von Eggeling, 1896; Forster, 1926; Elftman, 1932; Hill, 1962). 



Adaptations in the musculature of the upper extremity. — 

 The shoulder region of Alouatta is of great importance for the elucida- 

 tion of the locomotor changes imposed on this animal by the increase 

 in size of the structures related to the i)roduction of voice. An inter- 

 esting combination of muscular characteristics typical of the New 

 World brachiators (see Campbell, 1937) with other details indicative 

 of quadrupedal locomotion (see Ash ton and Oxnard, 1963) are to 

 be found in the pectoral girdle of the howler. Also, some of the shoulder 

 muscles which participate in the extension of the neck have under- 

 gone modifications. 



The trapezius is a muscle of the \'ago-accessory group of the head 

 and together with the caudal ])art of the serratus anterior, it is an 

 important elevator of the shoulder (see Oxnard, 1963). The muscle 



