96 THE VOYAGE OF H.M.S. CHALLENGER. 



abductor and short flexor proper to the little toe, and a ligament which arises from the 

 calcaneum." Chauveaux ' states that it corresponds to the two muscles, which in man, He 

 along side the interosseous metacarpal muscles. I am at a loss to understand the two 

 muscles to which he refers. Gamgee and Law 2 are also somewhat vague. They remark 

 that the interosseous muscle of the mediusis " transformed into the suspensory ligament." 

 Meckel in his " Traite General d'Anatomie Comparee " (p. 443), asserts that this ligament 

 is not only the representative of the interossei, but also of the lumbrical muscles. Two 

 minute lumbricals, however, are present as independent muscles in the foot of the horse. 3 

 Professor W. H. Flower, in his recent article upon the " Zoology and Anatomy of 

 the Horse," in the ninth edition of the Encyclopaedia Britannica, vol. xii. p. 178, says : — 

 " Its attachments and relations, as well as the occasional presence of muscular fibres in its 

 substanee, show that it is the homologue of the interosseous muscles of other mammals, 

 curiously modified, both in structure and function to suit the requirements of the horse's 

 foot." 



The second (PI. IX. fig. 7, d s ) and third dorsal interossei (PI. IX. 7, d*) are very 

 rudimentary, and are placed one upon each side of the flexor brevis or suspensory ligament. 

 Each muscle consists of a small fleshy belly, about two inches in length, succeeded by a 

 long narrow delicate tendon. The second dorsal interosseus arises from the outer side of 

 the base of the small second metatarsal bone, whilst the third springs from the inner side 

 of the base of the rudimentary fourth metatarsal and each is inserted upon its own side 

 of the fetlock joint, where it joins the band sent by the suspensory ligament to the 

 extensor tendon on the dorsum of the first phalanx. 



The dorsal interossei of the Horse, from their minute size, can exercise no abducting 

 action upon the medius. They are merely vestiges, and point to retrograde development. 

 Eigot considers that they have " the power of raising the synovial membranes of the 

 pastern joint and sesamoid sheath during flexion " (vide Gamgee and Law, p. 413). A close 

 study of their connections renders such an action very improbable. 



The chief interest of these rudimentary interossei muscles centres in the fact that they 

 constitute a link in the soft parts between the Horse of the present day, and its three-toed 

 ancestor. They are undoubtedly vestiges of well developed interosseous muscles which 

 lay in the second and third intermetatarsal spaces, and exercised the usual abducting action 

 upon the middle digit. Occasionally they are found greatly enlarged. Thus in a Horse, 

 which was dissected last winter session (1880-81) in Dick's Eoyal Veterinary College, 

 Edinburgh, Mr. M'Fadyean, the Professor of Anatomy, informs me that the fleshy bellies 

 of these muscles were several inches long and proportionately thick ; indeed, each exceeded 

 in size a strongly-marked plantaris muscle in man. 



1 Comparative Anatomy, Fleming's Translation, p. 154. 



2 hoc. cit., p. 413. 



3 Vide Gamgee and Law, he. cit., p. 413 ; Ckauveau's Comparative Anatomie, p. 311. 



