Henry Gray (18251861). Anatomy of the Human Body. 1918.
one to three or four in number; entering the fibrous capsule, they divide several times, and, losing their medullary sheaths, ultimately end in naked axis-cylinders encircling the intrafusal fibers by flattened expansions, or irregular ovoid or rounded disks (Fig. 939). Neuromuscular spindles have not yet been demonstrated in the tongue muscles, and only a few exist in the ocular muscles.
FIG. 939 Middle third of a terminal plaque in the muscle spindle of an adult cat. (After Ruffini.) (See enlarged image)
2. The Common Integument
(Integumentum Commune; Skin)
The integument(Fig. 940) covers the body and protects the deeper tissues from injury, from drying and from invasion by foreign organisms; it contains the peripheral endings of many of the sensory nerves; it plays an important part in the regulation of the body temperature, and has also limited excretory and absorbing powers. It consists principally of a layer of vascular connective tissue, named the corium or cutis vera, and an external covering of epithelium, termed the epidermis or cuticle. On the surface of the former layer are sensitive and vascular papillæ within, or beneath it, are certain organs with special functions: namely, the sudoriferous and sebaceous glands, and the hair follicles.
The epidermis, cuticle, or scarf skin is non-vascular, and consists of stratified epithelium (Fig. 941), and is accurately moulded on the papillary layer of the corium. It varies in thickness in different parts. In some situations, as in the palms of the hands and soles of the feet, it is thick, hard, and horny in texture. This may be in a measure due to the fact that these parts are exposed to intermittent pressure, but that this is not the only cause is proved by the fact that the condition exists to a very considerable extent at birth. The more superficial layers of cells, called the horny layer (stratum corneum), may be separated by maceration from a deeper stratum, which is called the stratum mucosum, and which consists of several layers of differently shaped cells. The free surface of the epidermis is marked by a net-work of linear furrows of variable size, dividing the surface into a number of polygonal or lozenge-shaped areas. Some of these furrows are large, as opposite the flexures of the joints, and correspond to the folds in the corium produced by movements. In other situations, as upon the back of the hand, they are exceedingly fine, and intersect one another at various angles. Upon the palmar surfaces of the hands and fingers, and upon the soles of the feet, the epidermal ridges are very distinct, and are disposed in curves; they depend upon the large size and peculiar arrangements of the papillæ upon which the epidermis is placed. The function of these ridges is primarily to increase resistance between contact surfaces for the purpose of preventing slipping whether in walking or prehension. The direction of the ridges is at right angles with the force that tends to produce slipping or to the resultant of such forces when these forces vary in direction.1 In each individual
Note 1. Professor Arthur Thomson, of Oxford, suggests that the contraction of these muscles on follicles which contain weak, flat hairs will tend to produce a permanent curve in the follicle, and this curve will be impressed on the hair which is moulded within it, so that the hair, on emerging through the skin, will be curled. Curved hair follicles are characteristic of the scalp of the Bushman. [back]