MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 



173 



Table 3. — Comparison of m. serratus anterior between Alouatta and locomotor 

 groups {data for semibrachiators and quadrupeds taken from Ashton and 

 Oxnard, 196S; for Alouatta, this work). 



Significance of difference in mean numbers of ribs spaimed by m. serratus anterior, pars caudalis; semi- 

 brachiators and quadrupeds: p< 0.001. 



a rather constant extensor of the neck in Alouatta. A caudal limit 

 for its spinal insertion on T4 places the howler in the modal value 

 for the quadrupeds of Ashton and Oxnard (1963) (table 4). This 

 figure, I should add, corresponds to one end of the range of their 

 semibrachiators (T4-T8). The atlantoscapularis posterior has been 

 often described as the pars cranialis of the serratus anterior. I agree 

 that in the howling monkey the muscle might be regarded as merely 

 the highest digitation of the serratus; but because of its scapular 

 attachment and the presumed importance which the muscle might 

 have in Alouatat as an extensor of the neck, I decided to treat it 

 separately. The traction exerted by the atlantoscapularis posterior 

 on its supraspinous anchorage might be the factor responsible for the 

 peculiarly extensive fossa in Alovutta. 



The partly quadrupedal and brachiating characters of the howler 

 are very evident in those muscles which act as propulsors of the upper 

 extremity. The cranial limit for the origin of m. latissimus dorsi puts 

 Alouatta in the region of overlap between quadrupeds (T4-T8) and 

 the semibrachiators (T6-T10) (table 5). The expansion of the mus- 

 cular attachment into the rib cage averages four slips in seven speci- 

 mens of Alouatta (4 mine; 3 of Sirena, 1871). The average in Cehus 

 is four ribs (see Schiick, 1913a). The number in Lagothrix is not clear 

 but Brachyteles appears to have six (Hill, 1962), and Ateles six (Schiick, 

 1913a). The development of this muscle in the howler did not impress 

 me as robust, but the almost perpendicular direction of its ventral 

 fibers reminds one of the condition in Ateles (table 1). The origin of 

 m. teres major occupies a large area near the angle, a character 

 attributed by Ashton and Oxnard (1963) to the muscle in quadrupeds 

 (table 1). The strong development of the triceps brachii bears mtness 

 to the importance of this muscle in the extension of the arm. 



