358 



AN INTRODUCTION TO ZOOLOGY 



CLASSIFICATION OF THE TISSUES. 



A. ECTODERMAL. 



1 . Epidermis : 



(a) Epidermal append- 

 ages, hair, scales, 

 etc. 



(b) Skin glands (sweat, 

 sebaceous, mam- 

 mary, lachrymal). 



2. Epithelium of : 



(a) Conjunctiva. 



(b) Olfactory chamber 

 (neuro-epithelium). 



(c) Auditory organ 

 (neuro-epithelium). 



(d) Oral cavity, oral 

 organ, enamel or- 

 gan, salivary glands, 

 part of hypophysis. 



(e) Anus, rectal glands. 

 (/) Chorion. 



Amnion. 

 Lens. 



3. Nervous tissue. 



(a) Cells of brain, 

 spinal cord and 

 sympathetic 

 ganglia. 



(b) Retina of eye. 



(c) Ependymal epithe- 

 lium. 



(d) Part of hypophysis. 



(e) Pineal body. 



B. MESODERMAL. 



1 . Mesothelium : 



(a) Epithelium of peri- 

 cardium, perito- 

 neum, pleura, uro- 

 genital organs and 

 kidney. 



(b) Striated muscles. 



2 . Mesenchyme : 



(a) Connective tissue, 

 non-striate muscle, 

 adipose tissue, and 

 pigment cells. 



(6) Supporting tissue, 

 cartilage, bone, liga- 

 ment, bone marrow. 



(c) Spleen. 



(d) Blood, blood- 

 vessels, endothe- 

 lium. 



(e) Eye-parts except 

 lens, retina, and 

 conjunctiva. 



(/) Framework of 



gonads. ,.. 



C. ENTODERMAL. 



1. Notochord. 



2. Epithelium of : 



(a) Digestive canal, 

 glands (gastric, in- 

 testinal, liver and 

 pancreas), allan- 

 tois. 



(b) Pharynx, Eusta- 

 chian tubes, tonsils, 

 thymus, thyroid. 



(c) Respiratory tract, 

 larynx, trachea, 

 lungs. 



Cytology. 



As we have seen, when we study the details of any tissue we are 

 brought down to the cell as the ultimate particle in almost all cases. 

 The cells are the fundamental vital units, and, indeed, in some cases, 

 e.g. the corpuscles of the blood and lymph, appear to be almost 

 separate organisms, only dependent on the body to a certain extent. 

 Just as the cell c*an be regarded as the structural unit, so it must also 

 be looked upon as the physiological unit, for all questions of function 

 ultimately can become questions of the physical and chemical 

 changes occurring in the cell. In the case of the lowest animals, 

 the Protozoa, the cell is in all senses the living unit, for each cell 

 constitutes a separate individual capable of manifesting all the vital 

 phenomena. As we ascend the animal scale, however, we find, in 

 the cell aggregates we call the Metazoa, that the capabilities of 

 individual cells become limited, and we have a division of labour 

 brought about whereby certain functions are confined to certain 



