Of extreme importance is the law of the relative distribution of motor and sensory filaments derived from one nerve-trunk. This, Van der Kolk's law,[1] has hardly received the attention it deserves from practical neurologists. Briefly stated, it is that of the two sorts of fibres in a mixed nerve the sensory filaments go to those parts which are moved by muscles innervated by the motor filaments of the same nerve. The reader can verify for himself the exactness of this law by making sketches of an extremity and tracing the motor and sensory distribution of its various nerves. There are partial and apparent exceptions to the formula, but this objection applies to almost all our medical laws. In the cranial system of nerves it is necessary to consider the trigeminus as the sensory companion of the six anterior motor nerves; the pneumogastric as the associate of the spinal accessory (in part). [Footnote 1: Van der Kolk, _On the Minute Structure and Functions of the Spinal Cord, etc._, p. 7, New Sydenham Soc. transl., London, 1859; Hilton, _On Rest and Pain_, p. 101, Am. ed., N. Y., 1879.] (_a_) Irritative lesions of nerves, as tumors, punctured wounds, perineuritis, moderate pressure, etc., are indicated by pain, numbness, and other paræsthesiæ in their cutaneous distribution, and of spasm or cramp with paresis only, in the associated muscles. Pain is the most prominent symptom by far, and many cases of so-called neuralgia belong to this category. As a rule, there is no anæsthesia, and the electrical reactions of nerves and muscles remain normal, or at least they do not present De R. In some cases vaso-motor spasm (coldness, white or bluish appearance of parts) shows itself in the most peripheral distribution of the nerve. The cutaneous and tendinous reflexes are variable, but usually preserved. (_b_) Destructive lesions of nerves, by section, severe pressure, true neuritis, etc., are characterized by anæsthesia with or without paræsthesiæ, by motor and vaso-motor paralysis, and by loss of superficial and deep reflexes. Later, there occurs degenerative atrophy of the paralyzed muscles with fully-developed De R., and dystrophic changes in the skin, etc. supplied by the sensory filaments of the injured nerve. In these anæsthetic parts ulceration is easily caused by traumatism and want of cleanliness. The abnormal electrical reactions of the paralyzed muscles are of much importance for the diagnosis of the amount of injury done to the nerve and for the purpose of prognosis. They may be summarized as follows, assuming a case of complete section of a nerve-trunk: