The immediate agent of expulsion in defx cation, micturition, and parturition is the inhe rent contractility of the muscular coat of the proper organ. Being hollow muscles, the sti mulus of distension is well adapted to excite them to contract. The will exercises consider able power in defiecation and micturition, both upon the rnuscular fibres of the viscera themselves, and on the abdominal muscles. In parturition the voluntary contraction of these latter muscles may give some assistance, but the main force of expulsion is due to the con traction of the uterine muscular fibres. In all the three actions, hovvever, the influence of the rnuscular fibres of the viscera respectively en gaged may be materially promoted by the con tractions of the abdominal muscles which are partly voluntary and partly reflex, being excited by the pressure of the mass to be expelled on the sensitive nerves in the neighbourhood, which, acting on the spinal cord, stimulate the muscular nerves, and through them cause the muscles they supply to contract, in harmony with the muscular tunic of the expelling viscus, rectum, bladder, or uterus, as the case may be.
I may here remark, that whilst it is suffi ciently evident that expulsion of the semen is a physical or reflex act, it cannot be admitted that erection of the penis is essentially so in its ordinary mode of production. This act is one of emotion—a simple emotion of the mind is sufficient to develope it : it may, however, be developed by the application of a stimulus to the penis or scrotum, when it clearly partakes of the character of a reflex act, although even under these circumstances it would be incor rect to say that emotion had no influence in its production. It is well known, however, that in cases where the spinal cord has been severely injured, severed indeed, by fracture and dis placement of some of the vertebrs, erection of the penis may be produced, although the organ is insensible, and the influence of the mind over the lower half of the body is suspended, and that even a slight stimulus as the friction of the bedclothes, or the introduction of a catheter, is sufficient for this purpose. This is clearly a purely reflex act, wholly independent of sensa tion or emotion ; but it may be likewise pro duced or kept up by the irritated state of- the cord itself. The painful erection of the penis, called chordee, which occurs in cases of inflam matory gonorrhcea, is partly a reflex phenon • non, but is chiefly due to a change in the circulation of the penis, to an increased attrac tion of blood to the organ in virtue of the intlamndatory state.
Enough has been said to shew that, to lay it down that every act of ingestion, of retention, of expulsion, and of exclusion, is a reflex act, is opposed to all that we know of the intimate nature of these actions. The power resident, not in the spinal cord only, but in every ner vous centre in which nerves are implanted, whereby, to use Prochaska's words, sensory impressions may be converted into motor im pulses, is no doubt of iminense importance to the animal economy; but Dr. M. Hall has been evidently led, by an imperfect analysis of the functions we have been considering, to assign to this power too large an influence in them; and on the other hand, he has over looked its obvious and important influence in other phenomena.
Dr. Marshall Hall also attributes to the spinal cord a direct action or influence which mani fests itself, first, in the tone, and secondly, in the irritability of the muscular system.
I regret to be compelled to differ again from Dr. IIall with respect to this point, and to express my opinion that this dogma is incon sistent with established doctrines of physiology.
By the tone of the muscular system, I un derstand that state of passive contraction which every healthy muscle exhibits when not in active contraction. It is this state which gives the firm, resisting, resilient feel, which the physician knows to be characteristic of a healthy state of the muscle. By virtue of it a muscle can adapt itself to changes which may take place in the distance between its two points of attach ment; and it is in virtue of this property that a muscle shortens itself when the stretching force of its antagonist has been removed. When the muscles of one side of the face have been paralysed for some short time, the features lose their balance, because the muscles of the sound side have contracted to within a smaller space, having lost the resistance of those of the opposite side. It is equality of tone which preserves the equilibrium between symmetrical muscles ; it is tone or passive contraction which keeps hollow muscles quite closed, if they are empty, or firmly contracted on their contents, if not so, as the heart and intestine; the tone of the predominant flexor muscles keeps linibs, whilst at perfect rest, in a semi flexed position ; it is tone which keeps sphinc ter muscles in a closed state.
The question is, do the muscles derive their state of tone frotn the spinal cord, and is this property dependent on that organ ? This question is answered in the negative, if we can shew that there are good and sufficient grounds for affirming that muscles possess within themselves all the conditions necessary for the generation of their proper force. That muscles do enjoy these conditions is manifest from the following considerations : 1. their peculiar chemical composition, their main con stituent being fibrine, a substance which, we know from tile phenomena of the coagulation of the blood, exhibits a remarkable tendency to contract ; 2. their anatomical constitution ; the arrangement in fibres, the intimate texture of those fibres, which in the muscles of the greatest power, the voluntary muscles, is highly complicated ; 3. from the large quantity of blood sent to muscles, which are probably more freely supplied with that fluid than any other texture in the body, and which receive it in the greater quantity when that contractile power is more active; 4. from the fact pointed out by Mr. Bowman, that a single muscular fibre, en tirely deprived of all nerves, may be made to contract by a slight stimulus applied to any part of it; 5. from the knowledge which we now possess that the mechanism of these ac tions may be seen by the microscope even in detached portions of muscular fibres ; 6. from the fact that muscles dissociated from the ner vous centres by the section of all the nerves distributed to them, retain their power of' con traction for a very considerable period, long after the nerves which sink into them have lost their excitability.