1881.] On the Tendinous Intersection of the Digastric. 33 



digastric muscles of opposite sides, extends between their anterior 

 bellies, covering the mylo-hyoid and part of the genio-hyoid muscles, 

 as the similarly placed muscular expansion in Gymnura and in Tupaia. 



In the species of the genus JErinaceus (the only other genus in- 

 cluded with Gymnura in the family PJrinaceidce), a superficial oblique 

 tendinous intersection occupies precisely the same position as in Gym- 

 nura, but is much less developed ; it extends from the upper margin 

 of the muscle (which is nearly of the same calibre from its origin to 

 its insertion), appearing on both surfaces as an oblique tendinous 

 inscription, and on reaching its lower margin is continued inwards 

 into a fascial expansion extending between the anterior bellies of the 

 muscles of opposite sides, covering the mylo-hyoid muscle, but no 

 muscular fibres arise from it, as in Gymnura. In Gentetes ecaudatus and 

 Hemicentetes madagascariensis, in Pteropus edulis and medius, and pro- 

 bably in all the species of that genus, in Epomophorus franqueti 

 (referred to above), in Megaderma lyra, in Phoca communis, and in 

 Gavia aperea, for instance, though the fascial expansion may be either 

 absent or feebly marked, the transverse tendinous inscription is trace- 

 able, although its presence in some, as in Gentetes ecaudatus, is indicated 

 only by a faint superficial oblique line surrounding the muscle in the 

 usual position.^ In these species, in which the anterior bellies of the 

 digastrics are not expanded and united between the jaws, the mylo- 

 hyoid muscles are well developed. 



The peculiar development of the anterior bellies of the digastric 

 muscles in the species referred to above, and the relations between it 

 and the tendinous intersection, demonstrate that whatever form this 

 intersection may assume, whether that of a rounded tendon, tendinous 

 band traversing the substance of the muscle, or feeble superficial ten- 

 dinous inscription, it has evidently been originally developed in 

 ancestral forms in the same manner, namely, as the point of attach- 

 ment of a tendinous raphe extending inwards from the digastrics of 

 opposite sides, and giving a fixed point from which the greatly ex- 

 panded internal laminae of their anterior bellies take their origin. In 

 most species of mammals the anterior bellies are no longer united 

 between the jaws, but the presence in many of a transverse tendinous 

 intersection or inscription still indicates their original connexion. 



While it appears evident that the original form and relations of the 

 tendinous intersection of the digastric, if not in all mammals, at least 



* Professor Humphry thus describes it in Phoca communis and in Cavia 

 aperea : — " Near its middle it presented in the seal a superficial transverse tendi- 

 nous division which is probably the representative of the more distinct tendinous 

 division in man ;" in the latter species, " where the muscle passes further forwards 

 nearer to the symphisis, the tendinous division is still more marked, involving the 

 greater number of the fibres, yet the muscle has nearly a straight course from its 

 origin to its insertion." — " Journ. Anat. Phys.," ii, p. 320. 



VOL. XXXII. D 



