a correct System of Muscular Homologies. 321 



animals already noticed as instances of a greater degree of 

 complexity than usual — the Hyrax^ Nycticebus^ &c. ; and in 

 all these we merely see approximation to the true type, in va- 

 rious degrees of distinctness. The long muscles in Hyrax are 

 probably displaced dorsal interossei, and the short inter- 

 phalangeal muscles of Nycticeius may belong to the same 

 type. 



The last class of muscles in the manus which are specially 

 devoted to produce the movements of the digits is that of the 

 dorsal interossei. These we usually find to be bicipital ; and 

 accepting this appearance as an evidence of the coalescence of 

 two muscle-germs, we can easily allocate these muscles to their 

 respective places. As extension is a much simpler act than 

 flexion, the latter being liable to endless modifications in 

 grasping, &c., so extensor muscles are much less disposed to 

 vary than flexors. Taking, again, the hand of man as an ex- 

 ample, we find the radial dorsal muscle of the poUex present 

 as the abductor pollicis exterior of Sommerring. The ulnar 

 dorsal muscle constitutes the polliceal origin of the first dorsal 

 interosseus, whose thumb insertion is obsolete. For the index 

 finger the radial dorsal muscle is developed as the first dorsal 

 interosseus ; this muscle in Macacus nemestrinus has no polli- 

 ceal ; but it is bicipital in Simla and in the hyaena and dog. 

 The ulnar dorsal muscle constitutes the outer head of the se- 

 cond interosseus. For the middle finger the dorsal radial 

 muscle forms the medial head of the second dorsal interosseus, 

 and the ulnar makes the corresponding head of the third dorsal 

 of the human hand. In the ring-finger the radial muscle is 

 modified into the annular origin of the third dorsal interosseus, 

 and the ulnar constitutes the annular origin of the fourth. For 

 the little finger the radial muscle forms the ulnar head of the 

 fourth dorsal interosseus, and the ulnar forms the opponens 

 minimi digiti. 



The pes exhibits an equally regular series. For the hallux 

 the dorsal tibial muscle is developed as the second head of the 

 abductor pollicis, and the dorsal fibular as the inner head of 

 the first dorsal interosseus. The second toe has its tibial muscle 

 in the form of the outer head of the first external interosseus, 

 and its fibular as the inner head of the second dorsal : this 

 muscle differs from its fellow in the manus in being inserted 

 into the second instead of the third finger. For the third toe 

 the tibial muscle constitutes part of the second dorsal inter- 

 osseus, and the fibular that part of the third which is attached 

 to the third metatarsal bone. Similarly the two dorsal muscles 

 for the fourth toe constitute respectively parts of the third and 

 fourth external interossei. For the little toe the tibial muscle 



