14 OUTLINES or EQUINE ANATOMY. 



the greater part, and consist of transparent fibres marked 

 by a number of nuclei ; they are smaller in calibre and 

 their nuclei more crowded than those of unstriated muscular 

 fibre. The white fibres in the living and very fresh subject 

 are transparent, resembling extremely minute glass threads, 

 but shortly after death they undergo change and exhibit a 

 central transparent but tough substance, axis cylinder, 

 surrounded by a thin layer of simple membrane, medullary 

 sheath, or neurilemma, while between the two is a quantity 

 of white albuminoid matter, which undergoes coagulation, 

 and is termed the ivhite substance of Schivann. These fibres, 

 with a few grey fibres, are united together to form those 

 white bands commonly known as nerves. Some of them seem 

 to commence in the tissues by free extremities, others by 

 loops, many in cells which will be found most marked 

 in the organs of special sense and in voluntary muscles. 

 In the latter we find these cells under the form of expanded 

 plates termed the motorial 'plates ; the rods and cones of 

 the eye, fibres of Corti of the ear, and Pacinian corpuscles 

 of the skin, are cells of special sense. Nerve-cells are found 

 in the nerve-centres, they are sometimes spheroidal, but 

 frequently present one, two, or more prolongations from 

 which the axis cylinders of the nerves pass. They are 

 termed polar cells {multi-, hi-, according to whether they 

 have many or but two prolongations). They abound in 

 the grey portions of the centres, the white being mainly 

 composed of nerve-fibres, connective tissue, and granular 

 matter. Nerve-cells present each a nucleus with one or 

 more nucleoli surrounded by granular matter sometimes of 

 a dark colour.* 



We have seen examples of pure membrane or lemma in 

 the investing layers of the nerve and muscular elements 

 termed respectively neurilemma and sarcolemma. A third 

 form is brought under our notice in investigating the com- 

 plex membranes of the body. It is termed basement mem- 

 brane, and consists of a thin structureless or slightly 

 granular fibre on one side in contact with connective tissue 

 with numerous vessels, on the other with cells with more 

 or less marked walls, nuclei, and nucleoli, termed epithelial 



* When a number of nerves passing from different parts of a centre 

 become mixed at one part, and from this nerves, each composed of 

 fibres from several parts uf the centre, run to distinct organs, we term 

 the arrangement a plexus. 



