114 DISEASES OF CATTLE. 



Along its entire length run two fissures, one above and the other be- 

 low, exactly in the middle line, nearly dividing the cord in two lateral 

 halves. The cord is white externally and gray internally. Between 

 each two of the bones forming the spinal column the cord gives off a 

 pair of nerves, one nerve emerging from either side of the column. These 

 nerves (the spinal nerve) arise from the cord by two roots; the superior 

 root contains sensory fibers, and the inferior root gives the motor 

 fibers. The union of these roots forms a common nerve, which soon 

 divides into two branches, containing motor and sensory fibers; the 

 superior branch to supply the muscles and skin above, and the inferior 

 branch to supply the parts below, including fibers to form the sympa- 

 thetic division. The spinal cord conducts nervous impressions to the 

 brain and impulses from the brain, and is therefore a conductor of 

 both afferent and efferent currents. It also contains nerve centers, both 

 reflex and automatic. 



The fibers that convey motor impulses decussate or cross from one 

 side tt> the other in the part of the brain called the medulla oblongata; 

 therefore a motor impulse going from the right side of the brain crosses 

 over to the left side in the medulla oblvngata and is carried down the 

 left side of the spinal cord ; and in like manner, a motor impulse from 

 the left side of the brain is carried down the right side of the spinal 

 cord. 



The nerves that convey sensory impressions go immediately to the 

 opposite side of the spinal cord ; therefore, an impression of pain re- 

 ceived on the left hind leg, or any part of the left side of the body, is 

 conveyed by the sensory nerve fibers to the spinal cord and passes over 

 to the right side, and is conveyed to the brain by fibers on the right 

 side of the spinal cord. Thus it follows that if a lateral half of the 

 spinal cord be cut. all parts posterior to the cut on the same side will 

 be paralyzed, and ail parts on the side opposite to the cut will be de- 

 prived of sensation. 



The weight of the spinal cord in a cow of average size is estimated 

 to be 7 1 ounces. The spinal canal is continuous anteriorly with the 

 cranial cavity. 



The cranial cavity, formed by bones of the head, is irregular in shape, 

 and contains the brain or encephalon. 



The brain is continuous with the spinal cord; there is nothing to 

 mark the place where one leaves off and the other begins. Looking at 

 the external surface of the brain, on its superior aspect, the larger 

 mass the cerebrum is seen to be divided by a longitudinal fissure in 

 the median line into two equal parts, called the cerebral hemispheres, 

 behind which is a smaller mass called the cerebellum, resting on the 

 part called the medulla oblongata, which is continuous with the spinal 

 cord. On the under surface of the brain, between the medulla oblongata 

 and the cerebral hemispheres, there is a prominent part called the_po* 

 Varolii, which consists of transverse fibers running across from one 



