37 6 Proceedings of Eoyal Society of Edinburgh. [sess. 
differ widely from one another. Primer Bey, writing of human 
hair, describes three kinds of hair differing in this character, 
according as to whether there is a central canal devoid of medullary 
substance, a canal filled with medulla, or whether the hair is com- 
posed of cortical substance throughout. Eeissner * * * § refers to the 
partial absence of medullary substance in some animals, and its 
total absence in seals and some Chiroptera. Ridewood, f in a recent 
paper, draws attention to its absence in sloths, and quotes Welcher, 
who first noticed this fact. Poulton | states that the medulla is 
wanting in the slender unpigmented base and also in the ‘ neck ’ 
region in the hairs of Ornithorhynchus. Henle,§ who describes the 
medulla as a substance consisting as a rule of two rows of cells 
whose nuclei are flattened transversely, says that this substance is 
quite absent in the finer hairs, and is not constant in the stronger 
ones, failing here and there. Other authorities might be quoted to 
show the variability of the medulla in different animals’ hairs. 
In all the equine hairs that I have examined, even in the very 
finest, the medulla is present, though its degree of development is 
somewhat variable. It is usually absent for a considerable dis- 
tance, both from the point and from the base of the hair, and may 
have broken down in an unaccountable fashion in one or more 
places on the hair shaft. Moreover, it consists, at least in its 
thickest part, of certainly more than two rows of cells, the nuclei 
of which can be seen in suitably stained sections. They are not 
shown in the figures illustrating this paper, which are drawn from 
unstained preparations. The absence of the medulla at the base 
of the hair is accompanied in many cases by the absence of pig- 
ment in the cortex. This is well shown in the hairs of the Somali 
zebra, which will be described lower down. Such an absence is 
invariable in fully grown hairs. Hairs which have not yet grown 
to their full length retain the medulla to a point much nearer the 
root. This shows that with the growth of the hair, the medulla 
* Reissner, Be it rage zur Kentniss der Haare , Breslau, 1854. 
t Ridewood, “ On the Structure of the Hairs of Mylodon listai” Q.J.M.S., 
vol. xliv. 
X Poulton, “The Structure of the Bill and Hairs of Ornithorhynchus 
■ paradoxus ,” Q.J.M.S., vol. xxxvi. 
§ Henle, Hand, der Eingeweidelehre, Braunschweig, 1873. 
