THE ELEMENTARY TISSUES 1 3 



section between the sides and nearly to the cross piece of the 

 ''H/' is in the ventral side of the spinal cord. The smaller 

 cleft, which is more difficult to find, is the dorsal (posterior) 

 median sulcus and is in the dorsal side of the cord. The ends 

 of either side of the ''H" are called dorsal (posterior) and 

 ventral (anterior) horns. These horns are the beginning of 

 the dorsal sensory, and of the ventral motor roots of the spinal 

 nerve and contain the principal synapse involved in spinal 

 reflex movements. The neurons, which consist of the nerve 

 cell and its various processes, are sensory or afferent in the 

 dorsal root; that is, they convey stimuli from the periphery 

 (skin usually) to the central nervous system; while the neurons 

 in the ventral root are motor or efferent, that is, they convey 

 impulses from the central nervous system to the muscles. 



(b) The peripheral nerve trunks have both myelinated 

 (sheathed) and unmyelinated nerve fibers. A cross section 

 of such a nerve trunk shows numerous circles with a deeply 

 stained center, the myelinated nerve fibers, and much smaller 

 deeply stained independent centers, the unmyehnated fibers, 

 intersperced between the myehnated fibers. 



E. LIQUID TISSUES 



Prepared slides of adult and embryonal rabbit blood or of 

 amphibian or avian blood should be available. Blood cells are 

 of two primary types: The erythrocytes (red or colored cells) 

 and leukocytes (white or colorless cells). Blood platelets will 

 not be considered here. 



Erythrocytes of the adult rabbit do not have a nucleus 

 normally, but are nucleated in the embryo. Young mammalian 

 red blood cells (erythroblasts) are nucleated and may appear 

 in blood of adults after severe hemorrhage. In adult amphib- 

 ians and birds the erythrocytes are normally nucleated. 



Leukocytes are of several kinds, but are usually much 

 larger than red blood cells, have one or more nuclei, which m.ay 

 be almost any form, and stain deeply. 



