MUSCULAK SYSTEM OF MAMMALIA. 



39 



Vertical section of the middle or functional digit of the fore- 

 foot of the Horse. nr\ 



From the tendon of the last muscle arises the ' anterior dilator 

 of the nostril,' fig. 11, t, which, acting upon the interior nasal 

 cartilage, powerfully expands the aperture of the nose. The 

 'orbicularis oris,' fig. 11, o, the 'levator labii superioris,' the 

 'elevator of the chin,' 15 



and the ' depressors of 

 the lower lip, and angle 

 of the mouth,' are well 

 developed. 



The anatomy of the 

 limbs of the Horse 

 would be incomplete 

 without a notice of the 

 structure of the terminal 

 segment of these best of 

 terrestrial locomotive or- 

 gans, in the perfection of 

 which the whole mecha- 

 nical force is concentrated 

 on a single hoof. 



The longitudinal section of the huge finger that forms the foot 

 or 'hoof of the horse, fig. 15, shows the structure of the three 

 phalanges — proximal 1, middle 2, and distal or ungual 4, with that 

 of the sesamoid, or nut-bone 3, adding to the lever-power of the 

 division of the tendon, 7, of the flexor profundus, going to the last 

 phalanx : the insertion of the tendon of the ' flexor sublimis,' 6, 

 and that of the tendon of the common ' extensor,' 5, are also shown. 

 The hoof-box of the ungual phalanx is denser at its periphery, 12, 

 than at its base, 10, but is not continuous over either surface ; the 

 former part is the ' wall,' the latter the ' floor ' of the horny or 

 ' insensible ' hoof. The wall, or ' external wall,' has the form of a 

 hollow cone obliquely truncate above, so that it is highest in front, 

 12, becoming vertical, and lower as it extends backward, losing 

 density, degenerating partly into the elastic tissue, 9, but being 

 mainly inflected inward, toward the centre of the sole, where it 

 blends with the horny ' floor,' and forms the ' internal wall : ' 

 this supports the superincumbent softer elastic tissue, and partly 

 that called the 'frog,' fig. 16, 3, for which a triangular space is 

 left between the inflected parts of the ' internal wall.' Thus the 

 posterior part of the periphery and of the floor of the ' hoof is 

 left uncovered by the horny box, which is accordingly free for a 

 certain degree of elastic expansion and contraction, especially 

 posteriorly. The inner surface of the ' wall ' is produced into a 



