PHYSIOLOGICAL ANATOMY OF THE NAILS. 385 



distinct. The Malpighian layer is applied directly to the ridges of the bed of the nail 

 and presents upon its upper surface ridges much less strongly marked than those of the 

 underlying true skin. This layer is rather thinner than the horny layer, is whitish in 

 color, and is composed of numerous strata of elongated, prismoidal, nucleated cells, 

 arranged perpendicularly to the matrix. These cells are from ^Vfr to T^TF f an ^ ncn m 

 length. 



The horny layer, which constitutes the true nail, is applied by its under surface 

 directly to the ridges of the Malpighian layer. It is dense and brittle and is composed of 

 numerous strata of flattened cells, which cannot be isolated without the use of reagents. 

 If the different strata of this portion of the nail be studied after boiling in a dilute solu- 

 tion of soda or potash, it becomes evident that here, as in the horny layer of the epider- 

 mis, the lower cells are somewhat rounded, while those nearer the surface are flattened. 

 These cells are nearly all nucleated and measure from -j-^Vo to T ^ of an inch in diame- 

 ter. The thickness of this layer varies in different portions of the nail, while that of 

 the Malpighian layer is nearly uniform. This layer is constantly growing, and it consti- 

 tutes the entire substance of the free borders of the nails. 



The connections of the nails with the true skin resemble those of the epidermis ; but 

 the relations of these structures to the epidermis itself are somewhat peculiar. Up to 

 the fourth month of foetal life, the epidermis covering the dorsal surfaces of the last 

 phalanges of the fingers and toes does not present any marked peculiarities ; but, at 

 about the fourth month, the peculiar hard cells of the horny layer of the nails make 

 their appearance between the Malpighian and the horny layer of the epidermis, and at 

 the same time the Malpighian layer beneath this plate, which is destined to become the 

 Malpighian layer of the nails, is somewhat thickened, and the cells assume more of an 

 elongated form. The horny layer of the nails constantly thickens from this time ; but, 

 until the end of the fifth month, it is covered by the horny layer of the epidermis. After 

 the fifth month, the epidermis breaks away and disappears from the surface ; and, at the 

 seventh month, the nails begin to increase in length. Thus, at one time, the nails are 

 actually included between the two layers of the epidermis ; but, after they have become 

 developed, they are simply covered at their roots by a narrow border of the horny 

 layer, the epidermis commencing again under the nail where the free border leaves the 

 bed. The nails are therefore to be regarded as modifications of the horny layer of 

 the epidermis, possessing certain anatomical and chemical peculiarities. The Malpighian 

 layer of the nails is continuous with the same layer of the epidermis, but the horny lay- 

 ers are, as we have seen, distinct. 



One of the most striking peculiarities of the nails is in their mode of growth. The 

 Malpighian layer is stationary, but the horny layer is constantly growing, if the nails be 

 cut, from the root and bed. It is evident that the nails grow from the bed, as their 

 thickness progressively increases in the body from the root to near the free border ; but 

 their longitudinal growth is by far the more rapid. Indeed, the nails are constantly 

 pushing forward, increasing in thickness as they advance. Near the end of the body of 

 the nail, as the horny layer becomes thinner, the growth from below is diminished. 



Hairs, varying greatly in size and development, cover nearly every portion of the 

 cutaneous surface. The only parts in which they are not found are the palms of the 

 hands and soles of the feet, the palmar surface of the fingers and toes, the dorsal surface 

 of the last phalanges of the fingers and toes, the lips, the upper eyelids, the lining of the 

 prepuce, and the glans penis. Some of the hairs are long, others are short and stiff, and 

 others are fine and downy. These differences have led to a division of the hairs into 

 three varieties : 



The first variety includes the long, soft hairs, which are found on the head, on the 

 face in the adult male, around the genital organs and under the arms in both the male 

 and the female, and sometimes upon the breast and over the general surface of the body 

 and extremities, particularly in the male. 

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