Part 5

=Spinal=: Atrophy in special muscular groups.

=Encephalic=: Little tendency to form bed sores.

=Spinal=: Tendency to form sloughs and bed sores.

Sensory (Æsthesodic) and Motor (Kinesodic) Tracts in Encephalon.

In the crus and above, the sensory tract lies dorso-laterad of the motor tract, forming about one-fifth of the crus, and extending upward through a white layer bending inward to form an angle and finally diverging to the different cortical convolutions. The motor tract is mainly contained in the inferior pyramids of the bulb, and constitutes the median two fifths and basal two fifths of the crus. Without entering farther into this subject it will be observed that lesions of the outer layer of the crus and its radiating fibres may cause hemianæsthesia of body or head, including the eye, while lesions of the median and basal layers and radiating fibres induce hemiplegia of the head, tongue, fore limb, hind limb, trunk, etc.

=Respiratory Centres, Inspiratory and Expiratory= are in the floor of the fourth ventricle between the centres for the vagus and accessory nerves, and are directly stimulated by the CO_{2} in the blood. Secondary subsidiary centres are in the optic thalamus, in the corpora quadrigemini both anterior and posterior pairs, and finally in the cervical spinal cord, so that disorder of respiration may occur from lesions in these points as well as in the main oblongata centre.

=Respiratory Inhibition= and arrest depend on the vagus, the superior and inferior laryngeal nerves.

=The Salivation Centre= also lies in the floor of the fourth ventricle and stimulation of the medulla causes free secretion.

=The Centres for Sneezing, Coughing, Sucking, Chewing, Swallowing and Vomiting= are also seated in the oblongata, so that any one of these phenomena may come from a central irritation. In bulbar paralysis the loss of power usually extends from the tongue through the lips, cheeks, jaws, pharynx, larynx, to the respiratory muscles and heart. Coughing may be roused by irritation of the external auditory meatus, liver, stomach, bowels, or generative organs as well as from the air passages.

=Cardiac Accelerating and Inhibiting Centres= are both present in the bulb, the latter receiving its afferent impulse mainly through the vagus nerve. Stimulation of the vagi, anæmia of the bulb through decapitation or through tying both carotids, hyperæmia through tying of the jugulars, a venous state of the blood, and blows on the abdomen all slow or arrest the heart action. Digitalis or muscarin has a similar effect. The heart action is accelerated by febrile and inflammatory affections, by a high or low temperature by section of the vagi, by sipping of cold water, by atropine or curari, and by salts of soda. Potash salts on the other hand restore the inhibitory action of the vagi and lower the heart’s action.

=The Vaso-Motor Center= is also in the oblongata and the contraction of the vessels with increase of arterial pressure may ensue from afferent currents in the sympathetic nerve and many sensory trunks. The varying activity is seen in blushing, in the congestion of mucous membranes under rage or excitement, in the capillary contraction in the early stage of inflammation, in the second stage of capillary dilation, in angioma or nævus and in extensive congestions and hæmorrhages in different organs. The arrest of bleeding under fainting is due largely to the anæmia of this centre.

=A Spasm Centre= the pricking of which causes general convulsions lies in the medulla oblongata at its junction with the pons. This is excited by excess of carbon dioxide in the blood, by suffocation, drowning, by anæmia of the bulb from bleeding or ligature of the carotids, by venous congestion after ligature of the jugulars, or by the direct application to the part of ammonia carbonate, or salts of potash or soda. It may also be roused by afferent nervous currents from different peripheral parts (spinal cord, sciatic nerve, etc.).

=A Perspiratory Centre= is found in the medulla, on each side, which may be roused into action by diaphoretics (opium, ipecacuan, tartar emetic, Calabar bean, nicotin, picrotoxin, camphor, pilocarpin, ammonia acetate, etc.).

=The Pons= like the medulla is at once a ganglionic and conducting organ, and its lesions may lead to arrest of nerve currents generated above or below it, or to the failure to develop currents in its own centres. Stimulation of its superficial layers may be without effect, but if this is carried into the centre epileptiform convulsions ensue. Lesions of one side of its posterior half cause facial paralysis on the same side and motor and sensory paralysis on the opposite side of the body (crossed hemiplegia). Lesions of one side of its anterior half cause paralysis in both face and body on the same side. This depends on the crossing of the fibres midway back in the pons, which cross again in the medulla (motor fibres) and in the spinal cord (sensory fibres). Lesions of the pons are liable to interfere with the functions of the trigemini, the oculo motor and the superior oblique, and to determine epileptic movements and loss of coördination of sensorio-motor movements. Lesions of the superficial transverse fibres (median cerebellar peduncles) tend to cause involuntary movements to one side.

=Lesions of the Corpora Quadrigemina= cause disturbance of vision, failure of the pupil to contract to light, blindness, paralysis of the oculo-motor nerves, and lack of coördination of movements. Stimulation of one anterior corpus causes rolling of both eyes to the opposite side, with, if continued, a similar movement of the head and even of the body (horse in mill, or index motion, or rolling on its axis).

=The Crura Cerebri= are conducting bodies but contain also different nerve centres. Lesions of one crus cause violent pain and spasm on the opposite side of the body, followed by paralysis. The oculo motor may be paralyzed on the same side, but the face and tongue on the opposite side, owing to the fibres crossing in the pons. There may be turning movements.

=The Optic Thalamus= transmits sensory currents to the cerebral cortex. Lesions in this organ cause sensory paralysis on the opposite side of the body. Afferent currents that do not traverse the thalamus cause reflexes only. It contains one of the roots of the optic nerve and its destruction will impair vision. Its injuries may also produce turning movements.

=The Corpus Striatum= transmits motor currents originating in the cerebral cortex. Lesions of its interior (lenticular nucleus) cause motor paralysis and sometimes anæsthesia on the opposite side of the body. Electrical stimulation of this nucleus causes general muscular contractions of the opposite side of the body. Irritation of the surface layers is painless and symptomless.

=The Cerebellum= has been long credited with coördination, and Flourens, after its removal from a pigeon, found an utter lack of harmonized movement in walking, springing or balancing. Luciani removed the organ from a bitch and, after full healing of the part, found a lack of muscular tone (a cerebellar ataxy), so that no great muscular effort could be satisfactorily accomplished. After months, marasmus set in and proved fatal. The lack of coördination is especially connected with lesions of the vermiform process, those of the posterior portion causing falling forward and those of the anterior portion, falling backward. Injury to the middle peduncle on one side causes turning or rolling to the opposite side. Under slighter injuries there may be only unsteadiness and staggering like a drunken man. Nausea and vomiting, with more or less stiffness of the neck or oposthotonos, may be present. Rolling of the eyes or squinting may occur.

=Focal Cortical Centers of the Cerebrum. Cortical Localization.= Much has been done experimentally and by observation of morbid lesions to locate functions in the different convolutions, and though the subsidiary implication of adjacent and interdependent parts interferes with a perfectly confident diagnosis, yet certain fundamental facts may be borne in mind as contributing to a satisfactory diagnosis.

Arloing, on the basis of his own experiments and those of his predecessors, gives the following as applicable to the equine (ass) brain:

1. Stimulation of the origin of the front part of the first frontal convolution, or of the anterior part of the pre-Sylvian convolution, causes approximation of the feet on the opposite side of the body.

2. Stimulation of the superior part of the first frontal convolution or of the superior part of the post-Rolandic convolution causes closure of the jaws and diduction.

3. Stimulation of the anterior end of the upper orbital convolution, or of the anterior part of the pre-Rolandic convolution, leads to movements of the nose and upper lip.

4. Stimulation of the antero-superior part of the lower frontal convolution, or the union of the post-Rolandic with the Sylvian convolution causes movement of tongue and jaws.

5. Stimulation of the union of the vertical and horizontal parts of the orbital convolution or frontal lobe, causes opening of the jaws and bending of head and neck.

6. Stimulation in the front of the union of the frontal and longitudinal convolutions, or at the union of the Sylvian and second parietal convolution causes rolling of the opposite eye.

7. Stimulation of union of the frontal and parietal parts of second parietal convolution leads to closure of both eyelids or, with a strong current, of lids on both sides.