OF NON-VASCULAR ANIMAL TISSUES. 187 



gether by a gelatinous substance. The harder part of the nail consists of compressed 

 and transparent corpuscles. 



x41though the nails do not contain any blood-vessels, they change their colour and 

 become friable under certain conditions of the circulation, thus showing that their 

 component cells have the power of circulating through them the fluid which is brought 

 to them by the blood-vessels, at their attached surface. 



The nails are in contact with the vascular chorion at two points ; their attached 

 margin is inserted into a groove of the chorion, and the attached surface lies upon 

 that portion which covers the dorsum of the terminal phalanges. 



The vessels which have the office of supplying the nail with a nutrient fluid are 

 large and numerous. The arteries take their origin from the digital trunks, and they 

 converge towards the dorsal surface of the terminal phalanx, on which they ramify, 

 and where they may be considered to terminate in two sets of vessels; one of which 

 is devoted to the supply of the ungual groove, the other to the ungual surface of the 

 phalanx. The ungual groove is very vascular ; it presents the ramifications of arteries, 

 which, after division and subdivision, form large plexuses in the margin of that part 

 of the groove which overlaps the nails, and the arteries terminate in loops of consi- 

 derable size. The vessels of the ungual surface are of a considerable size ; they form 

 large convolutions ; these give off^ branches, which, with others from the interior and 

 lateral part of the phalanx, form an intricate capillary network, which is in immediate 

 opposition with the attached surface of the nail. 



The Claws and Horns of quadrupeds, and the claws and beaks of birds, have a struc- 

 ture very analogous to the nails of the human subject. They are in contact, at their 

 attached surface, with large vessels. In some instances the bone upon which they rest 

 is perforated by foramina, the chorion subjacent to them, and between them and the 

 bone, is very attenuated, and they appear to be nourished also by the vessels con- 

 tained in the bone itself. 



Hoofs can be considered only as condensations of the cuticle. In foetal Calves, at 

 an early period of their existence, the hoof is not thicker than the cuticle of the heel 

 of the adult human subject, to which it is analogous in its structure. In adult age 

 that part of the hoof which is in connection with the chorion retains its analogy with 

 the cuticle, but at its free surface it becomes hard and somewhat friable. It is im- 

 possible for me here to enter into the details of the structural varieties of the hoofs 

 of animals, so as to point out any peculiarities in their composition which shall assist 

 in diftusing the nutrient fluid through the whole of their substance, to endow it with 

 the elasticity essential to the due performance of its functions. I will content myself 

 with stating, that it appears to me that the use of the elongated tubes containing white 

 soft matter, and which pervade the substance of the hoof of the Horse, is to convey 

 through its substance the fluid secreted at its attached surface. 



MDCCCXLI. 2 c 



