102 THE VOYAGE OF H.M.S CHALLENGER. 



arises from the base of the fourth metatarsal, and is inserted into the extensor tendon on 

 the outer aspect of the base of the annular digit. The inner or tibial head is represented 

 by a very delicate fibrous slip inserted into the inner sesamoid. 



The flexor brevis medii and flexor brevis annularis, lie directly under cover of the 

 two adductor muscles, and are separated from them by the deep divison of the external 

 plantar nerve. 



The fibular head of the flexor brevis minimi digiti is the sole representative of this 

 muscle, and as it is traversed throughout its whole length by a tendinous intersection, 

 it is in all probability in combination with the abductor minimi digiti, which is absent as 

 an independent muscle. It arises from the ligamentous textures at the base of the meta- 

 tarsus, and is inserted upon the outer side of the base of the first phalanx of the minimus 

 partly into the sesamoid, and partly into the extensor tendon. 



Dorsal layer — In the Pig's foot we find no muscle belonging to this layer. The 

 second and fourth dorsal interossei are converted into powerful ligamentous flattened 

 bands which lie concealed in the second and fourth inter-metatarsal spaces. The former is 

 inserted upon the inner face of the base of the medius partly into the sesamoid, and 

 partly into the extensor tendon, and the latter is inserted similarly upon the outer aspect 

 of the base of the first phalanx of the annularis. It is possible that the fibrous cord 

 inserted into the outer sesamoid of the medius may represent the third dorsal interosseous 

 muscle, but of this there is no decided proof. 



But to what does this disposition, and, in some cases transformation, of the intrinsic 

 muscles point ? The short marginal digits are alone capable of lateral movement. They 

 alone, therefore, retain their adductors, whilst their flexores breves are placed in such a 

 position that when acting alone they exercise an abducting influence, and when acting 

 in unison with the adductors they produce flexion. 



With regard to the largely-developed annularis and medius in which lateral 

 movement is prohibited, the function of the muscles usually set aside for this purpose is 

 changed. Functionally we may look at these two digits as in reality one, and the 

 muscles are arranged in accordance with this. The dorsal interosseus on each side of 

 this double digit is converted into a strong hgamentous band. These two bands 

 together act in a manner exactly similar to the suspensory ligament of the Horse, Ox, or 

 Sheep ; they prevent over-extension at the metatarso-phalangeal joints. The fibrous band 

 inserted into the outer sesamoid of the medius also contributes feebly to produce this 

 effect. 



Of the four fleshy bellies which typically represent the flexores breves of the medius 

 and annularis only two are retained, viz., the marginal bellies {i.e., outer head of flexor 

 brevis annularis and inner head of flexor brevis medii), and these together act as a short 

 flexor for the double digit. 



