Page:The New International Encyclopædia 1st ed. v. 14.djvu/421

NERVOUS SYSTEM AND BRAIN. reprssent descending branches from posterior root fibres, by others, descending axione of cells sit- uated in the cord. The descending cerebellar tract, or tract of Loewenthal, consists, as its

TRANSVERSE SECTION OF THE SPIRAL CORD OF A CHICK ON THE 9TH DAY OF INCUBATION.

a. Asia cylinders of anterior root fibres issuing from large cells of the anterior horn (c); b posterior root fibres passing from the bi-polar cells of the spinal ganglion into the posterior column of the spinal cord; d, when they bi- furcate, and e, when they become longitudinal: f and g, collaterals from the fibres passing into the gray matter.

name intimates, of descending axones of neurones whose colls are situated in the cerebellum. The results of investigations as to the paths which these axones follow in passing from cerebellum

SECTIONS OF THE SPINAL COUD, IN THE LOWER CERVICAL, MID-DORSAL, AND MID-LUMBAR REGIONS ON THE RIGHT SIDE THE CONDUCTING TRACTS ARE INDICATED).

Nerve cells;a meslal group: b lateral or ventro-lateral group; c. dorsal or dorso-laterai group: d, lateral cell column group; e. Clark's column group.

to cord are not in accord. It is probable that in the ease of many of the fibres two neurones are involved, the connection being made through the olivary nucleus of the medulla. These axones end in the gray matter of the cord. The colmun of Goll and the column of Burdach are com- posed of the axones of the cells of the ganglia of the posterior roots of the spinal nerves. Entering the posterior columns, these axones divide into as- cending and descending arms. The latter is short and soon ends in the gray matter. The ascending arm may be short, of medium length, or long. The short and medium length fibres end in the gray matter of the cord, after sending in collaterals at different levels. The long gibres pass up to the medulla and end. those of the column of Goll in the nucleus gracilis, those of the column of Burdach in the nucleus euneatus. The column of Lissauer consists of the shortest fibres which enter the posterior roots. These fibres end in the posterior horns. These neurones, whose central axones form the entering fibres of the posterior roots of the spinal nerves taken in connection with the analogous sensory roots of the cranial nerves, constitute the peripheral sensory neurone system. It is by means of these neurones that all impulses are conveyed from the periphery or sur- face of the body to the cord and brain. By their influence over the motor cells of the cord they determine the direct reflex actions. By their in-

LARGE GANGLION CELL OF THE VENTRAL HORN OF THE SPINAL CORD. ''Ax. cyl. pro., axis cylinder process; Nuc. reti.'', nucleo- reticulum; Nucls, nucleolus; ''Nuc. m.'', nuclear membrane; Cyto reti., cyto-reticulum; ''Chro. b.'', chromophilic bodies: ''Proto. pro.'', protoplasmic processes.

fluence over other cells situated in the cord, me- dulla, and mid-brain, sensory impressions are transmitted through other systems of neurones to the brain, and thus enter into consciousness. By their influence upon motor cells of the cortex they complete the cycle of a sensory impression en- tering consciousness and being transformed into a voluntary motion. The direct cerebellar tract or dorso-lateral ascending tract lies near the lat- eral surface of the cord from the posterior horn to about the mid-lateral point. It begins in the upper lumbar region and extends to the cerebel- lum. Its fibres are axones of the cells of Clark's column or the nucleus dorsalis of the same side. This nucleus is a group of cells lying in the gray matter Just to the median side of the base of the posterior horn. It extends as a continuous column from the mid-lumbar to the lower cervi- cal region. In the medulla the direct cerebellar tract enters the restiform body and thus reaches the cerebellum by means of the inferior cerebel- lar peduncle, Gower's tract, or the ventro-lateral ascending tract, lies along the periphery of the cord from the anterior limit of the direct cere- bellar to the anterior nerve roots. The tract first appears in the upper lumbar cord and in- creases in size as it passes upward. Its fibres are axones of cells scattered through the gray matter