88 THYSIOLOGY OF FARM ANIMALS [CH. 



hairs which grow permanently, not being shed at recurrent 

 periods. Such are the hairs of the mane and tail (with some 

 exceptions), the eyelashes, and the long tactile hairs on the 

 muzzle. The comparatively short hairs of the upper jDart of the 

 tail (i.e. those forming the tail-lock) in Icelandic ponies and 

 other ponies of the Celtic type are shed at the begmning of 

 summer and regrown at the commencement of the following cold 

 season. These hairs, as Ewart has pointed oiit, serve the purpose 

 of protecting the anal region from snow which collects upon the 

 upper surface of the tail-lock. It is well known that clijoping 

 or cutting has a stimulating effect upon hairs, causing them to 

 grow longer. In some animals there are muscles attached to the 

 hair roots, the function of which is to ruffle the hairs and so 

 diminish the conduction of heat. Similar muscles are connected 

 with the feathers of birds. 



The hairs of various kinds of animals present considerable 

 differences. Thus the hairs in deer are composed almost entirely 

 of medulla; those in pigs consist largely of horny substance; 

 while hair which has the property of mutual cohesion or 'felting,' 

 depending upon a rough scaly surface and a disposition to curl, 

 is characteristic of the sheep, being commonly called 'wool.' 



Nails and daws like hairs are formed from the corneous cells 

 of the epidermis Avhich instead of being thrown off as flakes are 

 consolidated to form horny structures. Underneath the nail is 

 the vascular nail bed which is a" modified part of the dermis 

 thrown up into parallel ridges. The hoofs of Ungulates (de- 

 scribed below in the chapter dealing with the organs of loco- 

 motion) have a similar origin to nails, being developed from 

 horny epidermal elements fitting on to a modified dermis. The 

 horns of cattle and sheep consist of bony processes ensheathed 

 in a case of true horn which like the structures mentioned above 

 is of epidermal origin. 



Sweat glands. Over most parts of the epidermis in man and 

 in the horse but confined to more or less restricted areas in certain 

 other animals (see p. 91) are numerous pores which represent 

 the openings of the ducts of the sweat glands. These ducts 

 convey the secretions of the glands to the surface of the skin, 

 and in so doing they traverse a portion of the dermis and the 

 whole of the epidermis. The actual glands are situated in the 

 dermis and consist of coiled tubes surrounded by a network of 



