56 U.S. NATIONAL MUSEUM BULLETIN 2 73 



18b), a fine subdivision is also sent in front of that muscle in the lower 

 fourth of the abdomen. 



According to Sirena (1871), the aponeurotic laminae of both the 

 internal oblique and the transversus abdominis behave as in man in 

 the formation of the rectus sheath. 



Nerve supply : These last two abdominal muscles are innervated by 

 the lateral cutaneous branches of the thoracoabdominal intercostal, 

 subcostal, and iliohypogastric nerves. 



Function: The broad abdominal muscles are important in suj)- 

 l)orting the viscera while the animal stands or moves on his four ex- 

 tremities. In addition, they are also used in expiration, defecation, 

 parturition, and micturition. 



Comparative anatomy: These muscles have in general the same 

 arrangement in all the five genera under consideration. There seems to 

 be, nevertheless, considerable variability among them in the formation 

 of the rectus sheath (see Hill, 1962; Mijsberg, 1915; Miller, 1947; 

 Tschachmachstschjan, 1912; Winckler, 1950). 



M. cremaster: The fibers of this muscle form a conspicuous layer 

 over the spermatic cord and the testicles. They arise from the caudal 

 free border of both the internal oblique and the transversus as these 

 muscles form the roof of the inguinal canal. Other fascicles come 

 medially from the pubis. 



Nerve supply: The genital branch of tlie genitofemoral nerve. 



M. dartos: It is included in this section for topographical reasons. 

 It is represented by conspicuous and abundant fibers which develop in 

 the membranous layer of the superficial abdominal fascia and accom- 

 pany its extensions over the penis and testicles. Its nerve supply could 

 not be determined. 



Subvertebral Group 



M. longus capitis (fig. 19) : This is a thick muscular column which, 

 from its origin on the external surface of the basioccipital, descends 

 covered by the prevertebral fascia. Short and tendinous fascicles leave 

 its deep aspect to be inserted on every anterior tubercle and the 

 transverse process of T 1 . 



M. longus colli: It lies mostly under cover of the longus capitis 

 and has three parts (fig. 19). The superior oblique portion arises by 

 tendinous digitations from the anterior tubercle of C 7 up to C 3. 

 There was always a slip from T 1 . These form a flat and fleshy belly 

 which runs craniomedially to insertion on the tubercle of the anterior 

 arch of the atlas. The inferior oblique portion has muscular origins 

 from the intervertebral disks (not the bodies) between T 4 and 1. 

 Insertion is tendinous upon the anterior tubercles of C 7 to C 5. The 

 vertical portion lies between the other two and the midline. It is rep- 

 resented by a series of mostly tendinous bands which arise from 



