TEGUMENTARY SYSTEM. 3 



The two are often very distinct, so that, as in the spines of the 

 hedgehog, a canal is found internally, separated by transverse parti- 

 tions into cellular intervals. Sometimes, on the contrary, particularly 

 in the cervine Ruminantia (the Roe), the cortical substance appears 

 entirely wanting, and the hair is made up throughout of coarser cells. 

 The cortical substance is generally coarser and harder than the me- 

 dullary, but frequently passes insensibly into it. The latter is in many 

 cases wanting, as almost always in the hair of the human head, where 

 epithelium and cortical substance are alone found, while in other sit- 

 uations, as upon the chin, the eyelashes, eyebrows, nose, axillae, and 

 pubis, the hairs possess a medullary centre. Most hairs are not 

 round, but compressed upon one or two sides, so as to present a 

 transversely oval section (Dasyprocta), or one that is kidney-shaped 

 (GirafTa), or that is quadrangular (Histrix Javari.), or irregularly an- 

 gular (Auchenia Llama). 



The hairs upon their external surface are for the most part smooth 

 and even, as in Man, or they exhibit slight lateral projections, as in 

 the Squirrel, or they are knotty, as in the Bear, or provided with 

 pointed processes like the teeth of a saw, which in some cases 

 (Mygale) stand out only upon one side, in others (Pteropus) upon 

 both, or they are furnished with thorn-shaped processes, as in the 

 Cheiroptera. They are rarely found channelled, as is the ease in 

 the two-toed Sloth, by rounded longitudinal ridges and intervening 

 grooves. The gray and grayish- white hairs of such animals as the 

 Mole and Mouse, exhibit a variegated appearance, like the down of 

 Birds. They are annulated with black at regular intervals, where 

 the hair is either transparent, or else surrounded by more delicately 

 marked rings. The spines of the Hedgehog and Porcupine do not 

 essentially differ in structure from hair, they only seem to contain 

 more of the same materials. Their epithelium is very much de- 

 veloped, and the cortical substance consists of small, elongated cells, 

 and is of a horny consistency. The medullary tube is very spa- 

 cious, and contains two kinds of cells. In the different species of 

 the genus Erinaceus, we perceive differences in the form and size 

 of these internal cells. In bristles, e. g. of the Hog, there is found, 

 a very small compressed medullary tube, and in the cortical sub- 

 stance a very ample cellular structure. In the several orders of 

 Mammalia, very great differences occur, so that they can scarcely 

 be said to have anything in common. Thus all the Apes have three 

 substances, which vary much, however, in their relative preportions. 

 In the Carnivora the cortical substance appears always to predomi- 



