Struggles of the Platysma.

In interpreting this novel hair stream of man’s chest and neck we are again brought into an atmosphere of struggle of forces. Something has occurred in the course of man’s descent from the ape to interfere very sharply with the course of the hair; and certainly if there be anything in organisms that Heredity, Variation and Selection are unable to do (even when adorned with capital letters, to make them, as Huxley said, “like grenadiers with bearskins,” appear much finer fellows than they are), it is to provide in this reversed stream of hair on man’s chest some cunning “adapta­tion” to his needs. Selection will not serve; but I think use and habit will. There can be little, if any, doubt that the frequent and active contractions of the platysma muscle in the course of man’s life are the efficient cause of the change of arrangement of hair from a downward simian to an upward human slope. To this opinion the anatomist will promptly reply: “Ah! I have thee there, friend Lamarckian; are there not any number of apes and monkeys that also have an active and efficient platysma?” Undoubtedly there are, and I give here, through the kindness of Professor Keith, a short account of that muscle in simiadæ. It is taken from an unpublished work of his on The Myology of the Catarrhini—a Study in Evolution. The account may be only interesting to the professed anatomist, but the conclusions in the summary bear closely on the present problem. I give the exact words from Chapter II., pp. 472, 479. The simian forms examined are semnopithecus, gorilla, chimpanzee, orang, gibbon, macacus, cercopitheci, cynocephali. “Summary: Every gradation is found between the cynocephalic and human forms. The evolution lies in the disappearance of the supra-trapezial origin and the superficial labio-mental insertion. The opposite nuchal and mental angles of a trapezoidal sheet are obliterated and a rhomboidal figure is left. The change may be seen step by step through the macaci, semnopitheci, hylobates, troglodytes and the orang to Man.

“The maxillary insertion in man is more extensive than the others, and the insertion is more distinctly demarcated from the quadratus menti origin. But slips between the two muscles are not uncommon.

“The sub-mental interdigita­tion occurs frequently in man, and although its extent varies in the other Catarrhini it is always present.

“The upper nuchal fibres, being cut loose in the higher members of the orthorachitial group from their primary origin, became aberrant in their behaviour. Auriculo-labial slips, slips of union with the zygomatici, or simulating a risorius, or a relapse to the primitive medial dorsal origin and connec­tion with the occipito-auricular muscles may occur in man as in the others.

“Fasciculation of the muscle may occur in man and the troglodytes.

“That the functions of this muscle are indefinite is shown by the numerous individual and generic variations. But that its presence is essential may be judged by its persistence. It may depress the angle of the mouth or the lower jaw, or help to flex the head upon the chest, or help to empty the laryngeal air-sac if it be present. But as a matter of fact all these functions are otherwise provided for. When tense it protects the deep part of the neck somewhat, and it is usually active in temper. The axillary part of the same sheet in the cynomorphæ offers a similar puzzle as regards its functions.”

We have it thus on the highest authority that the platysma muscle is active and persistent in a large series of monkeys, apes and man. But the whole work has for its sub-title, “A Study in Evolution,” and in the story of the platysma there is a picture of its progressive development to that of man. There is evidence in the above account of the muscle that a structure is found in monkeys and man which might operate on the overlying streams of hair in any of these animal forms—or might not—in accordance with the concep­tion of struggle between opposing forces which I have kept in view all through this volume.

It is evident that in all animals below man the platysma has not achieved any victory by its action over the streams of hair on the chest and neck, and to my mind it is equally evident that in the case of man it has carried through a very manifest “turning-movement.” It will be objected, quite properly, that this is a matter of opinion, and the pertinent question will be asked, “How do you account for the absence of this reversed hair-pattern in apes and monkeys and its absence in man, both having an efficient platysma muscle?”

The essence of a struggle is that it ends with the victory of one adversary over the other, and as the race is not always to the swift nor the battle to the strong, there is of necessity some uncertainty as to the result of any struggle. The factors of time as well as of overwhelming force are required for most of the victories of man over man, and it is not less so in the victories of habit over ancestry in the direction of hair, as I have repeatedly shown. The required time is clearly at one’s disposal for this victory, and the “overwhelming force” of habit and use is purely a question of the degree of repeti­tion and the efficiency of the contractions of the platysma, and its greater use in man than in apes and monkeys. The uses to which it was put in the lower forms not having been sufficiently overwhelming for victory, no change in them has been shown. The cumulative effects of the actions of a developing platysma in man, under the guiding influence of his more complex habits of life, have turned the scale in favour of the reinforced forces of habit, and the direction of the hair becomes reversed nearly all over the area covering the muscle.

We must consider all the forces engaged in this struggle for mastery on the neck and chest of man, and remember on one hand the power of the normal slope of hair, the greater difficulty of altering the direction of the thick long hairs of monkeys and apes, and their relatively long resting hours; and on the other the shorter and finer hairs of man and the increasing efficiency of his platysma muscle in varied actions. Professor Keith mentions four functions of the platysma: that of depressing the angle of the mouth and lower jaw, helping to flex the head upon the chest, and to empty the laryngeal air-sac, and protecting the deep parts of the neck when it is tense—adding the significant comment that “it is usually active in temper”—I presume this to mean bad temper!

Leaving out of account the emptying of the laryngeal air-sac, is it not evident that the remaining three actions of the platysma are very much more exerted in the case of man with all the numerous occupations and movements of his head and neck, in obedience to his higher brain, than in the apes, monkeys and lemurs, endowed with a fitful activity, with fewer and less variable movements of their head, and long, long hours spent in their particular form of medita­tion?

So, when the muscular sheet, which, as I have said, is closely attached to the skin of the chest and more loosely to that of the neck, contracts and becomes shortened between its origin on the chest to its insertion in the face and jaw, it gives a most obvious pull on the skin over it and wrinkles it vertically in a manner which will strike any thin person who contracts it voluntarily before a looking-glass. The connec­tion shown between the action of the platysma muscle and the change of hair is so close that it can hardly be questioned that one is the cause of the other. If it be not proved to demonstra­tion it is “tremendously probable” and the connec­tion falls into line with the previous demonstrated cases.

I must add here a remark suggested by the views of man’s descent put forward since this was written. The claim that man has changed the direction of his hair on his back and chest by use and habit owing to altered modes of life is not dependent on the simian theory of his descent. The change to his present patterns on those two regions from those of any “active arboreal pioneer” among insectivores is just as striking and is open to the same line of explana­tion.

It would serve no useful purpose here to travel further over the varied streams of hair on the body of man.

CHAPTER XIV.
MISCELLANEOUS EXAMPLES.

In this chapter a few of the rarer examples of hair-clad mammals which present remarkable changes at critical areas of their hairy coats may be considered with advantage. I have chosen six, of which three appeared in my former book.