In deglutition, the exciting cause is the sti mulus of contact applied to the mucous mem brane of the fauces. So highly sensitive is the mucous membrane in this situation, that the slightest touch of it with a feather is sufficient to produce contraction of the muscles of deglu tition, which the will is scarcely able to con trol. Without this stimulus, it is doubtful whether these muscles would obey the will alone, and it seems probable that this part of the act of deglutition must be regarded as one of those actions referred to at a former page, which require a double stimulus, both mental and physical, for their full performance.
The medulla oblongata and its continuations in the mesocephale appear to be the centre of those actions which are influenced by emotion. The common excitement of movements of de glutition or respiration, or of sensations referred to the throat, under the influence of emotion, evidently points to this part of the cerebro spinal centre as being very prone to obey such impulses; and as the nerves of pure sense, especially the optic, and auditory, are very commonly the channels of sensitive impressions well calculated to arouse the feelings, it seems highly probable that the centre of such actions should be eontiguous to the origin of these nerves. This office may be assigned to that region of the mesocephale which is in the vicinity of the quadrigeminal tubercles. It is not a little remarkable that the nerves which arise from this and the neighbouring parts are very readily influenced by emotions. Thus, the third and fourth pairs of nerves regulate the principal movements of the eyeballs, those especially which most quickly betray emotional excitement; and the portio dura of the se venth pair, the motor nerve of the face, is the medium through which changes of the counte nance are effected. It may be added, that the centre of emotional actions ought to be so situ ated that it might readily communicate with the centres of all the voluntary actions of the body, and with the iinmediate seat of the intellectual operations, as well as with the nerves of pure sense; and no part possesses these relations so completely as that now under exaniination.
In those diseases which mental emotion is apt to give rise to, many of the symptoms are referable to affection of the medulla oblongata.
In hysteria, the globus, or peculiar sense of suffocation or constriction about ,the fauces; in chorea, the difficulty of deglutition, the pew, liar movement of the tongue, the excited state of the countenance, the difficulty of articula tion, may be attributed to the exalted polarity of the centre of emotional actions. In hydro phobia this part is probably always affected, and frequently so in tetanus.
Certain gangliform bodies are connected with the upward continuations of the medulla ob longata, both in the brain and in the mesoce phale, which doubtless have proper functions. These are the corpora striate, optic thalami, and quadrigeminal bodies.
Corpora striata.—The anatomy of the cor pora striata and optic thalami, while it denotes a very intimate union between them, also shows so manifest a difference in their structural cha racters, that it cannot be doubted that they perform essentially different functions. In the corpora striata the fibrous matter is arranged in distinct fascicles of various sizes, many, if not all of which, form a special connection with its vesicular rnatter. In the optic thala mi, on the other hand, the fibrous matter forms a very intricate interlacement, which is equally complicated at every part. Innumerable fibres pass from one to the other, and both are con nected to the hemispheres by extensive radia tions of fibrous matter. The corpora striatzt, however, are connected chiefly, if not solely, with the inferior fibrous- layer of each crus cerebri; whilst the optic thalami are continuous with the superior part of each crus, which is situate above the locus niger.
It will be observed, then, that while these bodies possess, as a principal character in com mon, an extensive connection with the convo, luted surface of the brain, they are, in the most marked way, connected inferiorly with separate and distinct portions of the medulla oblongata; the corpora striata with the inferior fibrous planes of the crura cerebri and their continua tions, the anterior pyramids; and the optic thalami with the olivary columns, the central and probably fundamental portions of the me dulla oblongata. This anatomical fact must be taken as an additional indication that these gangliform bodies perform separate functions.