310 FUNCTIONS OF THE NEIIVOUS SYSTEM. 



a. It lias been pointed out by Messrs. Todd and Bowman (Physiological Anatomy, Vol. I. 

 p. 315), that the Spinal Cord of the male frog, at the season of copulation, naturally pos- 

 sesses a state of most extraordinary excitability. The thumb of each anterior extremity at 

 this season, becomes considerably enlarged; as is well known to Naturalists. "This enlarge- 

 ment is caused principally by a considerable development of the papillary structure of the 

 skin which covers it; so that large papillas are formed all over it. A male frog, at this 

 season, has an irresistible propensity to cling to any object, by seizing it between his anterior 

 extremities. It is in this way that he seizes , upon, and clings to the female ; fixing his 

 thumbs to each side of her abdomen, and remaining there for weeks, until the ova have been 

 completely expelled. An effort of the Will alone could not keep up the grasp uninterrupt- 

 edly for so long a time, yet so firm is the hold, that it can with difficulty be relaxed. What- 

 ever is brought in the way of the thumbs, will be caught by the forcible contraction of the 

 anterior limbs; and hence we often find frogs clinging blindly to a piece of wood, or a dead 

 fish, or some other substance which they may chance to meet with. If the finger be placed 

 between the anterior extremities, they will grasp it firmly; nor will they relax their grasp 

 until they are separated by force. If the animal be decapitated, whilst the finger is within 

 the grasp of its anterior extremities, they still continue to hold on firmly. The posterior half 

 of the body may be cut away, and yet the anterior extremities will still cling to the finger; 

 but immediately that the segment of the cord, from which the anterior extremities derive 

 their nerves, has been removed, all their motion ceases. This curious instinct only exists 

 during the period of sexual excitement; for at other periods the excitability of the anterior 

 extremities is considerably less than that of the posterior." 



402. Nerves of the Spinal System. The nerves which minister to the 

 functions of the Spinal Cord, conveying to it the impressions made on the 

 periphery, and transmitting its motor influence to the muscles, are not those 

 alone which are ordinarily designated as Spinal nerves; for several of those, 

 which pass forth through the base of the cranium, and which are commonly 

 described as Cephalic nerves, belong to the same category. The general 

 characters of the Spinal nerves, their mode of connection with the Spinal 

 Cord by two sets of roots, and the presence of ganglion upon the posterior 

 root, have already been adverted to ( 344). The anterior roots are usually 

 the smaller ; and this is particularly the case with those of the cervical nerves, 

 in which the posterior roots are of remarkable comparative size. In the First 

 Cervical or Sub-occipital pair, the anterior roots are sometimes wanting ; but 

 there is then a derivation of fibres from the Spinal Accessory, or from the 

 Hypoglossal, or from both. The two roots of the ordinary Spinal nerves 

 unite immediately beyond the ganglion, which is situated on the posterior one; 

 and the trunk thus formed separates immediately into two divisions, the an- 

 terior and posterior,' each of which contains both afferent and motor fibres. 

 These divisions, of which the anterior is by far the larger, proceed to the ante- 

 rior and posterior parts of the body respectively; and are chiefly distributed to 

 the skin and the muscles. The anterior branch is that which communicates 

 with the sympathetic nerve. 



403. The pair of nerves commonly designated as the Fifth of the Cephalic 

 series, or as the Trifacial, is the one which more nearly resembles the ordi- 

 nary Spinal nerves (as was long since pointed out by Sir C. Bell), than docs 

 any other of those originating within the cranium. It possesses two distinct 

 sets of roots, of which one is much larger than the other ; on the larger root, 

 as on the posterior and larger root of the Spinal nerves, is a distinct ganglion; 

 and the fibres arising from the smaller root do not blend with the others, until 

 after the latter have passed through this ganglion. The trunk of the nerve 

 separates, as is well known, into three divisions, the Ophthalmic, the Supe- 

 rior Maxillary, and the Inferior Maxillary ; and it can be easily shown, by 

 careful dissection, that the fibres of the smaller root pass into the last of these 

 divisions alone. When the distribution of this nerve is carefully examined, 

 it is found that the first and second divisions of it proceed almost entirely to 

 the skin and mucous surfaces; a very small proportion, only, of their fibres 

 being lost in the muscles : whilst of the branches of the third division, a large 

 number are distinctly muscular. Hence analogy, and the facts supplied by 



