686 COMPARATIVE ANATOMY 



scala tympani. The ventral or descending staircase of the cochlea. 



scala vestibuli. The dorsal or ascending staircase of the cochlea. 



scapula. The shoulder blade. The dorsal arm of the pectoral girdle. 



Schwann's sheath. The neurilemma of a nerve fiber. 



sclerotic (sclera). The tough white outer covering of the eye-ball. 



sclerotome. The part of the somite which forms the vertebral column. 



scrotum. The integumentary sac in which the testes of mammals are lodged. 



scute. A scale-like skeletal structure. 



sebaceous gland. A gland which produces a greasy secretion. 



secretin. An intestinal hormone which activates the pancreas. 



secretion. A product of cell metabolism which is thrown out as waste or is used by 



the organism in its normal activity. 

 sella turcica. The pituitary fossa in the sphenoid bone, 

 semen. The external secretion produced by the male in coitus. 

 semicircular canal. One of the ducts of the membranous ear used in equilibration, 

 semilunar valves. Crescentic valves at the root of the aorta and pulmonary artery 



which prevent blood from returning to the ventricles. 

 seminal vesicle, A secretory gland and reservoir for spermatozoa connected with the 



ductus deferens. 

 seminiferous tubule. One of the secretory tubules of the testis. 



septula testis. The connective tissue partitions which divide the testes into compart- 

 ments. 

 septiun pellucidum. A double membrane which lies in the median plane of the brain, 



between the corpus callosum and the fornix. 

 septum transversimi. A transverse partition which separates pericardial and peritoneal 



cavities of vertebrates. 

 serous gland, A gland which secretes a thin watery secretion (serum) . 

 Sertoli cells. Columnar cells of the seminiferous tubules, which serve as a base of 



attachment for spermatozoa. 

 serum. The clear liquid portion of the blood devoid of cells and of fibrinogen. 

 sessile. Fixed or attached. 

 sex. The differentiation of two kinds of reproductive cells and of the individuals which 



produce them, 

 shell membrane. A tough double membrane attached to the shell of the hen's egg. 

 sigmoid flexure. The bent lower part of the colon to the rectum. 

 Sinanthropus. The Peking species of fossil man. 

 sino-auricular node. A group of fibers at the root of the precava which in contraction 



serves as "pace-maker of the heart" beat. 

 sinus. A cavity or hollow space. 

 sinusoid. Relatively large anastomosing blood spaces lined by endothelium without 



adventitia. 

 solenocyte. An elongated flagellated excretory cell associated with the protonephridia 



of annelids. 

 somatic muscle. Derived from the somite or epimere in contrast with the visceral 



muscle which is derived from the h>T)omere. 

 somatopleure. The combined layers of ectoderm and parietal mesoderm, 

 somite. A segment of the dorsal or epimeric mesoderm. 



spermatogonia. Immature germ-cells which by enlargement form the primary sperma- 

 tocytes. 

 sphenopalatine ganglion, Meckel's ganglion of the autonomic and facial nerves. 

 splanchnopleure. The double embryonic layer formed by the visceral layer of mesoderm 



and the endoderm. 



