72 COMPARATIVE ANATOMY. 



of nail. Hoofs are claws with the terminal portions blunted 

 and rounded, and adapted to the support of the limb. (Hux- 

 ley.) Hollow horns, as in Ruminantia, are analogous to 

 claws, but solid horns, as in Rhinoceros, are without bony 

 support, and probably are the result of coalescence of hairs. 

 Two kinds of glands, sebaceous and sudoriferous. The 

 former, almost always in connection with hair bulbs, produce 

 a fatty secretion. The latter lie within the subcutaneous con- 

 nective tissue as simple tubes, the ducts from which pursue 

 a more or less wavy course to open on the skin. In most 

 mammals the tube is coiled at its commencement, but in ox 

 and dog it is straight. In the latter the glands upon the body 

 are rudimentary caeca, but upon the ball of the* foot they are 

 large. (Huxley.) 



XI. 



KEEVOUS SYSTEM. 



NERVE tissue is that peculiar product of organization which, 

 while the source of sensation, controls excito-motory and nu- 

 tritive acts. 



Two kinds recognized, cellular and commissural.* 

 The nerve cell is globular or oval, and, possessed of at 

 least one spherical nucleus, is generally unipolar or bipolar, 

 rarely multipolar. A collection of nerve cells constitutes a 

 ganglion. A commissure may be either a prolongation of the 

 cell wall, or a distinct structure, characterised in the higher 

 animals as a white thread-like band, having an axis fibre, en- 

 closed by a medullary sheath, and the whole supported by 

 basement membrane (neurilemma). A ' nerve' is a bundle 

 of commissures. Ganglia are supposed to generate nerve 

 power; commissures to convey it from ganglion to ganglion, 

 or to and fro from adjacent structures. The nerves, however, 



* By ' commissure ' is assumed a band which not only unites ganglion with 

 ganglion, or perceptive area with ganglion, but also associates ganglion with 

 motor area. 



